#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 2;
const int mod = 1e9 + 7;
const double eps = 1e-8;
inline int read() {
  int x;
  scanf("%d", &x);
  return x;
}
long long a[N], b[N];
int main() {
  int n = read();
  for (int i = 1; i <= n; i++) {
    a[i] = read();
  }
  while (1) {
    bool ok = 0;
    int id = 0;
    for (int i = 1; i <= n; i++) {
      if (i < n && a[i] == a[i + 1]) {
        b[++id] = a[i] + 1;
        ok = 1;
        i++;
      } else
        b[++id] = a[i];
    }
    n = id;
    for (int i = 1; i <= id; i++) a[i] = b[i];
    if (!ok) break;
    sort(a + 1, a + n + 1);
  }
  long long ans = 0;
  a[0] = -1;
  for (int i = 1; i <= n; i++) {
    ans += a[i] - a[i - 1] - 1;
  }
  cout << ans;
}
