codeforces 305 C. Ivan and Powers of Two

//codeforces 305 C. Ivan and Powers of Two
//2013-06-05-17.19
#include <stdio.h>
#include <set>
#include <algorithm>
#include <iostream>
using namespace std;
set<int> s;
int main()
{
    int n, a;
    while (scanf("%d", &n) != EOF)
    {
        s.clear();
        int maxn = 0;
        for (int i = 1; i <= n; i++)
        {
            scanf("%d", &a);
            while (s.count(a))
            {
                s.erase(a);
                a++;
            }
            s.insert(a);
            maxn = max(a, maxn);
        }
        printf("%d\n", maxn-s.size()+1);
    }
    return 0;
}