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.util.*;
import java.io.*;
import java.math.BigInteger;
public class cf1 {
    static long mod = (long)1e9 + 7;
    static long mod1 = 998244353;
    static FastScanner f;
    static PrintWriter pw = new PrintWriter(System.out);
    static Scanner S = new Scanner(System.in);
    static long x0; static long y0;
    static int inf = (int)(1e9) + 5;
    static int oo = Integer.MAX_VALUE;
    static double eps = (double)1e-4; 

    public static void solve() {
        int n = f.ni();
        
        char s[] = f.next().toCharArray();
        char t[] = f.next().toCharArray();

        int cnt1[] = new int[26];
        int cnt2[] = new int[26];

        for (int i = 0; i < n; ++i) {
            ++cnt1[s[i] - 'a'];
        }

        for (int i = 0; i < n; ++i) {
            ++cnt2[t[i] - 'a'];
        }

        boolean ok = true;

        for (int i = 0; i < 26; ++i) {
            ok &= ((cnt1[i] + cnt2[i]) % 2 == 0); 
        }

        if (!ok) {
            pn("No");
            return;
        }

        pn("Yes");

        int cnt = 0;
        StringBuffer ans = new StringBuffer("");

        while (true) {
            ok = true;

            for (int i = 0; i < n; ++i) ok &= s[i] == t[i];

            if (ok) break;
            boolean swapped = false;
    here:   for (int i = 0; i < n; ++i) {
                if (s[i] == t[i]) continue;

                for (int j = 0; j < n; ++j) {
                    if (i == j) continue;
                    if (s[j] == t[j]) continue;

                    // a a
                    // b b
                    if (s[j] == s[i]) {
                        // Swap s[j] & t[i]
                        char y = s[j];
                        s[j] = t[i];
                        t[i] = y;
                        ++cnt;
                        swapped = true;
                        ans.append((j + 1) + " " + (i + 1) + "\n");
                        break here;
                    }
                }
            }

            if (!swapped) {
        there:  for (int i = 0;  i < n; ++i) {
                    if (s[i] == t[i]) continue;

                    for (int j = 0; j < n; ++j) {
                        if (i == j) continue;
                        if (s[j] == t[j]) continue;

                        // a b
                        // b a

                        if (s[i] == t[j]) {
                            cnt += 2;
                            // Swap s[j] & t[j]
                            ans.append((j + 1) + " " + (j + 1) + "\n");
                            char y = s[j];
                            s[j] = t[j];
                            t[j] = y;

                            // Swap s[j] & t[i]
                            ans.append((j + 1) + " " + (i + 1) + "\n");
                            y = s[j];
                            s[j] = t[i];
                            t[i] = y;

                            break there;
                        }
                    }
                } 
            }
        }
        pn(cnt);
        p(ans);
    }   

    public static void main(String[] args)throws NumberFormatException , IOException {   
        init();
        boolean tc = true;
        int t = tc ? f.ni() : 1;
        while(t --> 0) solve();
        pw.flush(); 
        pw.close();  
    }
        
/******************************END OF MAIN PROGRAM*******************************************/
    public static void init()throws IOException{if(System.getProperty("ONLINE_JUDGE")==null){f=new FastScanner("");}else{f=new FastScanner(System.in);}}
    public static class FastScanner {
        BufferedReader br;StringTokenizer st;
        FastScanner(InputStream stream){try{br=new BufferedReader(new InputStreamReader(stream));}catch(Exception e){e.printStackTrace();}}
        FastScanner(String str){try{br=new BufferedReader(new FileReader("!a.txt"));}catch(Exception e){e.printStackTrace();}}
        String next(){while(st==null||!st.hasMoreTokens()){try{st=new StringTokenizer(br.readLine());}catch(IOException e){e.printStackTrace();}}return st.nextToken();}
        String nextLine()throws IOException{return br.readLine();}int ni(){return Integer.parseInt(next());}long nl(){return Long.parseLong(next());}double nd(){return Double.parseDouble(next());}
    }
    public static void pn(Object o){pw.println(o);}
    public static void p(Object o){pw.print(o);}
    public static void pni(Object o){pw.println(o);pw.flush();}
    static int gcd(int a,int b){if(b==0)return a;else{return gcd(b,a%b);}}
    static long gcd(long a,long b){if(b==0l)return a;else{return gcd(b,a%b);}}
    static long lcm(long a,long b){return (a*b/gcd(a,b));}
    static long exgcd(long a,long b){if(b==0){x0=1;y0=0;return a;}long temp=exgcd(b,a%b);long t=x0;x0=y0;y0=t-a/b*y0;return temp;}
    static long pow(long a,long b){long res=1;while(b>0){if((b&1)==1)res=res*a;b>>=1;a=a*a;}return res;}
    static long mpow(long a,long b){long res=1;while(b>0l){if((b&1)==1l)res=((res%mod)*(a%mod))%mod;b>>=1l;a=((a%mod)*(a%mod))%mod;}return res;}
    static long mul(long a , long b){return ((a%mod)*(b%mod)%mod);}
    static long adp(long a , long b){return ((a%mod)+(b%mod)%mod);}
    static int log2(int x){return (int)(Math.log(x)/Math.log(2));}
    static boolean isPrime(long n){if(n<=1)return false;if(n<=3)return true;if(n%2==0||n%3==0)return false;for(long i=5;i*i<=n;i=i+6)if(n%i==0||n%(i+2)==0)return false;return true;}
    static boolean isPrime(int n){if(n<=1)return false;if(n<=3)return true;if(n%2==0||n%3==0)return false;for(int i=5;i*i<=n;i=i+6)if(n%i==0||n%(i+2)==0)return false;return true;}
    static HashSet<Long> factors(long n){HashSet<Long> hs=new HashSet<Long>();for(long i=1;i<=(long)Math.sqrt(n);i++){if(n%i==0){hs.add(i);hs.add(n/i);}}return hs;}
    static HashSet<Integer> factors(int n){HashSet<Integer> hs=new HashSet<Integer>();for(int i=1;i<=(int)Math.sqrt(n);i++){if(n%i==0){hs.add(i);hs.add(n/i);}}return hs;}
    static HashSet<Long> pf(long n){HashSet<Long> ff=factors(n);HashSet<Long> res=new HashSet<Long>();for(Long i:ff)if(isPrime(i))res.add(i);return res;}
    static HashSet<Integer> pf(int n){HashSet<Integer> ff=factors(n);HashSet<Integer> res=new HashSet<Integer>();for(Integer i:ff)if(isPrime(i))res.add(i);return res;}
    static int[] inpint(int n){int arr[]=new int[n+1];for(int i=1;i<=n;++i){arr[i]=f.ni();}return arr;}
    static long[] inplong(int n){long arr[] = new long[n+1];for(int i=1;i<=n;++i){arr[i]=f.nl();}return arr;}
    static boolean ise(int x){return ((x&1)==0);}static boolean ise(long x){return ((x&1)==0);}
    static int gnv(char c){return Character.getNumericValue(c);}//No. of integers less than equal to i in ub
    static int log(long x){return x==1?0:(1+log(x/2));} static int log(int x){return x==1?0:(1+log(x/2));}
    static int upperbound(int a[],int i){int lo=0,hi=a.length-1,mid=0;int count=0;while(lo<=hi){mid=(lo+hi)/2;if(a[mid]<=i){count=mid+1;lo=mid+1;}else hi=mid-1;}return count;}
    static void sort(int[] a){ArrayList<Integer> l=new ArrayList<>();for(int i:a)l.add(i);Collections.sort(l);for(int i=0;i<a.length;++i)a[i]=l.get(i);}
    static void sort(long[] a){ArrayList<Long> l=new ArrayList<>();for(long i:a)l.add(i);Collections.sort(l);for(int i=0;i<a.length;++i)a[i]=l.get(i);}
    static void sort(ArrayList<Integer> a){Collections.sort(a);}//!Precompute fact in ncr()!
    static int nextPowerOf2(int n){int count=0;if(n>0&&(n&(n-1))==0)return n;while(n!=0){n>>=1;count += 1;}return 1<<count;} 
}