This problem is different from the easy version. In this version Ujan makes at most 2n swaps. In addition, k ≤ 1000, n ≤ 50 and it is necessary to print swaps themselves. You can hack this problem if you solve it. But you can hack the previous problem only if you solve both problems.

After struggling and failing many times, Ujan decided to try to clean up his house again. He decided to get his strings in order first.

Ujan has two distinct strings s and t of length n consisting of only of lowercase English characters. He wants to make them equal. Since Ujan is lazy, he will perform the following operation at most 2n times: he takes two positions i and j (1 ≤ i,j ≤ n, the values i and j can be equal or different), and swaps the characters s_i and t_j.

Ujan's goal is to make the strings s and t equal. He does not need to minimize the number of performed operations: any sequence of operations of length 2n or shorter is suitable.

Input

The first line contains a single integer k (1 ≤ k ≤ 1000), the number of test cases.

For each of the test cases, the first line contains a single integer n (2 ≤ n ≤ 50), the length of the strings s and t. 

Each of the next two lines contains the strings s and t, each having length exactly n. The strings consist only of lowercase English letters. It is guaranteed that strings are different.

Output

For each test case, output "Yes" if Ujan can make the two strings equal with at most 2n operations and "No" otherwise. You can print each letter in any case (upper or lower).

In the case of "Yes" print m (1 ≤ m ≤ 2n) on the next line, where m is the number of swap operations to make the strings equal. Then print m lines, each line should contain two integers i, j (1 ≤ i, j ≤ n) meaning that Ujan swaps s_i and t_j during the corresponding operation. You do not need to minimize the number of operations. Any sequence of length not more than 2n is suitable.

Example

Input


4
5
souse
houhe
3
cat
dog
2
aa
az
3
abc
bca


Output


Yes
1
1 4
No
No
Yes
3
1 2
3 1
2 3
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.Writer;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.*;
import java.io.*;
import java.math.*;
public class Main7{

static class Pair
    { 
        int x;
		int y;
        public Pair(int x, int y) 
        {     
            this.x=x;
			this.y=y;
        }     
        
    @Override
            public int hashCode() 
            { 
                final int temp = 14; 
                int ans = 1; 
                ans =x*31+y*13; 
                return ans; 
            } 
      
            // Equal objects must produce the same 
            // hash code as long as they are equal 
        @Override
        public boolean equals(Object o) 
        { 
            if (this == o) { 
                return true; 
            } 
            if (o == null) { 
                return false; 
            } 
            if (this.getClass() != o.getClass()) { 
                return false; 
            } 
            Pair other = (Pair)o; 
            if (this.x != other.x || this.y!=other.y) { 
                return false; 
            } 
            return true; 
        } 
            
    } 
static class Pair1{
	int x;
	int y;
	long z;
	Pair1(int x, int y, long z)
	{
		this.x=x;
		this.y=y;
		this.z=z;
	}
}
    
    public static long pow(long a, long b)
    {
        long result=1;
        while(b>0)
        {
            if (b % 2 != 0)
            {
                result=(result*a)%1000000007;
                b--;
            } 
            a=(a*a)%1000000007;
            b /= 2;
        }   
        return result;
    }
    public static long fact(long num)
    {
                long value=1;
                int i=0;
                for(i=2;i<num;i++)
                {
                    value=((value%mod)*i%mod)%mod;
                }
                return value;
            }
            public static long gcd(long a, long b)
            {
                if (a == 0)
                    return b;
                return gcd(b%a, a);
            }
            
    /*        public static long lcm(long a,long b)
            {
                return a * (b / gcd(a, b));
            }
        */    public static long sum(int h)
            {
                return (h*(h+1)/2);
            }
        /*   public static void dfs(int parent,boolean[] visited,int[] dp)
            {
                ArrayList<Integer> arr=new ArrayList<Integer>();
                arr=graph.get(parent);
                visited[parent]=true;
                for(int i=0;i<arr.size();i++)
                {
                    int num=(int)arr.get(i);
                    if(visited[num]==false)
                    {
                        dfs(num,visited,dp);
                    }
                    dp[parent]=Math.max(dp[num]+1,dp[parent]);
                }
            }
        //    static int flag1=0;
            static int[] dis;*/
            static int mod=1000000007;
            static ArrayList<ArrayList<Integer>> graph;
            static ArrayList<ArrayList<Integer>> g;
            
       /*    public static void bfs(int num,int size)
            {
                boolean[] visited=new boolean[size+1];
                Queue<Integer> q=new LinkedList<>();
                q.add(num);
                ans[num]=1;
                visited[num]=true;
                while(!q.isEmpty())
                {
                    int x=q.poll();
                    ArrayList<Integer> al=graph.get(x);
                    for(int i=0;i<al.size();i++)
                    {
                        int y=al.get(i);    
                        if(visited[y]==false)
                        {
                            q.add(y);
                            ans[y]=ans[x]+1;
                            visited[y]=true;
                        }
                    }
                }
            }
            static int[] ans;*/
            
            
            
            
            
            
            
            
            
        //    static int[] a;              
            public static int[] sort(int[] a)
            {
                int n=a.length;
                ArrayList<Integer> ar=new ArrayList<>();
                for(int i=0;i<a.length;i++)
                {
                    ar.add(a[i]);
                }
                Collections.sort(ar);
                for(int i=0;i<n;i++)
                {
                    a[i]=ar.get(i);
                }
                return a;
            }
			static class pqcomparator implements Comparator<Pair>
			{
				public int compare(Pair p1 ,Pair p2)
				{
					if(p1.y>p2.y)
					{
						return 1;
					}
					else
						return -1;
				}
			}
			static public void main(String args[])throws IOException
            {
				int t=i();
				StringBuilder sb=new StringBuilder();
				while(t-->0)
				{
					StringBuilder ans=new StringBuilder();
					int n=i();
					String s=s();
					String s1=s();
					char[] ch=s.toCharArray();
					char[] ch1=s1.toCharArray();
					ArrayList<Integer> ar=new ArrayList<>();
					int[] tp=new int[26];
					for(int i=0;i<n;i++)
					{
						if(s.charAt(i)!=s1.charAt(i))
						{
							tp[s.charAt(i)-'a']++;
							tp[s1.charAt(i)-'a']++;
							ar.add(i);
						}
					}
					int tt=0;
					for(int i=0;i<26;i++)
					{
						if(tp[i]%2==1)
							tt++;
					}
					if(tt>0)
					{
						pln("NO");
					}
					else
					{
						int count=0;
						for(int i=0;i<ar.size();i++)
						{
							if(count>=2*n)
								break;
							if(ch1[ar.get(i)]!=ch[ar.get(i)])
							{
								for(int j=i+1;j<ar.size();j++)
								{
									if(i!=j)
									{
										if(ch1[ar.get(i)]==ch1[ar.get(j)] && ch[ar.get(j)]==ch[ar.get(i)] && ch[ar.get(j)]!=ch1[ar.get(j)])
										{
											char c=ch[ar.get(i)];
											ch[ar.get(i)]=ch1[ar.get(j)];
											ch1[ar.get(j)]=c;
											ans.append((ar.get(i)+1)+" "+(ar.get(j)+1)+"\n");
											count++;
											break;
										}
									}
								}
								if(ch1[ar.get(i)]!=ch[ar.get(i)])
								{
									for(int j=i+1;j<ar.size();j++)
									{
										if(i!=j)
										{
											if(ch1[ar.get(i)]==ch1[ar.get(j)] && ch[ar.get(j)]!=ch1[ar.get(j)])
											{
												char c=ch[ar.get(i)];
												ch[ar.get(i)]=ch1[ar.get(j)];
												ch1[ar.get(j)]=c;
												ans.append((ar.get(i)+1)+" "+(ar.get(j)+1)+"\n");
												count++;
												break;
											}
										}
									}
									if(ch1[ar.get(i)]!=ch[ar.get(i)])
									{
										for(int j=i+1;j<ar.size();j++)
										{
											if(i!=j)
											{
												if(ch[ar.get(j)]!=ch1[ar.get(j)] && ch[ar.get(j)]==ch1[ar.get(i)])
												{
													count+=2;
													char c=ch[ar.get(j)];
													ch[ar.get(j)]=ch1[ar.get(j)];
													ch1[ar.get(j)]=c;
													ans.append((ar.get(j)+1)+" "+(ar.get(j)+1)+"\n");
													c=ch1[ar.get(j)];
													ch1[ar.get(j)]=ch[ar.get(i)];
													ch[ar.get(i)]=c;
													ans.append((ar.get(i)+1)+" "+(ar.get(j)+1)+"\n");
													break;
												}
											}
										}
									}
								}
							}
						}
						if(count>2*n)
						{
							pln("NO");
						}
						else
						{
							int tmp=0;
							for(int i=0;i<ar.size();i++)
							{
								if(ch[ar.get(i)]==ch1[ar.get(i)])
								{
									tmp++;
								}
							}
							if(tmp==ar.size())
							{
								pln("YES");
								pln(count+"");
								System.out.print(ans.toString()+"");
							}
							else
							{
								pln("NO");
							}
						}
					}
				}
				System.out.print(sb.toString());
			}
			
            /**/
            static InputReader in=new InputReader(System.in);
                static OutputWriter out=new OutputWriter(System.out);
                public static long l()
                {
                    String s=in.String();
                    return Long.parseLong(s);
                }
                public static void pln(String value)
                {
                    System.out.println(value);
                }
                public static int i()
                {
                    return in.Int();
                }
                public static String s()
                {
                    return in.String();
                }
    }
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
         
        class InputReader {
             
            private InputStream stream;
            private byte[] buf = new byte[1024];
            private int curChar;
            private int numChars;
            private SpaceCharFilter filter;
         
            public InputReader(InputStream stream) {
                this.stream = stream;
            }
         
            public int read() {
                if (numChars== -1)
                    throw new InputMismatchException();
                if (curChar >= numChars) {
                    curChar = 0;
                    try {
                        numChars = stream.read(buf);
                    } catch (IOException e) {
                        throw new InputMismatchException();
                    }
                    if (numChars <= 0)
                        return -1;
                }
                return buf[curChar++];
            }
         
            public int Int() {
                int c = read();
                while (isSpaceChar(c))
                    c = read();
                int sgn = 1;
                if (c == '-') {
                    sgn = -1;
                    c = read();
                }
                int res = 0;
                do {
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    res *= 10;
                    res += c - '0';
                    c = read();
                } while (!isSpaceChar(c));
                return res * sgn;
            }
         
            public String String() {
                int c = read();
                while (isSpaceChar(c))
                    c = read();
                StringBuilder res = new StringBuilder();
                do {
                    res.appendCodePoint(c);
                    c = read();
                } while (!isSpaceChar(c));
                return res.toString();
            }
         
            public boolean isSpaceChar(int c) {
                if (filter != null)
                    return filter.isSpaceChar(c);
                return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
            }
         
            public String next() {
                return String();
            }
         
            public interface SpaceCharFilter {
                public boolean isSpaceChar(int ch);
            }
        }
         
        class OutputWriter {
            private final PrintWriter writer;
         
            public OutputWriter(OutputStream outputStream) {
                writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
            }
         
            public OutputWriter(Writer writer) {
                this.writer = new PrintWriter(writer);
            }
         
            public void print(Object...objects) {
                for (int i = 0; i < objects.length; i++) {
                    if (i != 0)
                        writer.print(' ');
                    writer.print(objects[i]);
                }
            }
         
            public void printLine(Object...objects) {
                print(objects);
                writer.println();
            }
         
            public void close() {
                writer.close();
            }
         
            public void flush() {
                writer.flush();
            }
         
            }
         
            class IOUtils {
         
            public static int[] readIntArray(InputReader in, int size) {
                int[] array = new int[size];
                for (int i = 0; i < size; i++)
                    array[i] = in.Int();
                return array;
            }
         
            } 

