Ivan has got an array of n non-negative integers a1, a2, ..., an. Ivan knows that the array is sorted in the non-decreasing order. 

Ivan wrote out integers 2a1, 2a2, ..., 2an on a piece of paper. Now he wonders, what minimum number of integers of form 2b (b ≥ 0) need to be added to the piece of paper so that the sum of all integers written on the paper equalled 2v - 1 for some integer v (v ≥ 0). 

Help Ivan, find the required quantity of numbers.

Input

The first line contains integer n (1 ≤ n ≤ 105). The second input line contains n space-separated integers a1, a2, ..., an (0 ≤ ai ≤ 2·109). It is guaranteed that a1 ≤ a2 ≤ ... ≤ an.

Output

Print a single integer — the answer to the problem.

Examples

Input

4
0 1 1 1


Output

0


Input

1
3


Output

3

Note

In the first sample you do not need to add anything, the sum of numbers already equals 23 - 1 = 7.

In the second sample you need to add numbers 20, 21, 22.
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import java.util.TreeSet;
public class cf305c {
  public static void main(String[] args) {
    FastScanner in = new FastScanner(System.in);
    int n = in.nextInt();
    TreeSet<Integer> set = new TreeSet<Integer>();
    for(int i=0; i<n; i++) {
      int x = in.nextInt();
      while(set.contains(x)) {
        set.remove(x);
        x++;
      }
      set.add(x);
    }
    int last = -1;
    int ans = 0;
    for(int x : set) {
      ans += x-last-1;
      last = x;
    }
    System.out.println(ans);
  }
  static class FastScanner {
    BufferedReader br;
    StringTokenizer st;
    
    public FastScanner(InputStream in) {
      br = new BufferedReader(new InputStreamReader(in));
      scanLine();
    }
    public void scanLine() {
      try {
        st = new StringTokenizer(br.readLine().trim());
      } catch(Exception e) {
        throw new RuntimeException(e.getMessage());
      }
    }
    public int numTokens() {
      if(!st.hasMoreTokens()) {
        scanLine();
        return numTokens();
      }
      return st.countTokens();
    }
    public String next() {
      if(!st.hasMoreTokens()) {
        scanLine();
        return next();
      }
      return st.nextToken();
    }
    public double nextDouble() {
      return Double.parseDouble(next());
    }
    public long nextLong() {
      return Long.parseLong(next());
    }
    public int nextInt() {
      return Integer.parseInt(next());
    }
  }
}
