#include <bits/stdc++.h>   using namespace std;   int n, a[100005], top, s[100005];   int main() {     ios_base::sync_with_stdio(0);     cin.tie();     cout.tie();     cin >> n;     for (int i = 0; i < n; ++i) cin >> a[i];     top = -1;     for (int i = n - 1; i >= 0; --i) {       s[++top] = a[i];       while (top > 0 && s[top] == s[top - 1]) ++s[--top];     }     cout << s[0] - top;     return 0;   }   