#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, lam, mx = 0, c = 0;
  cin >> n;
  vector<int> cand(n);
  for (int i = 0; i < n; i++) {
    cin >> cand[i];
    if (mx < cand[i]) mx = cand[i];
  }
  for (int i = 0; i < n; i++)
    if (mx == cand[i]) c++;
  int x = lam = cand[0];
  if (lam == mx && c > 1) return cout << "1", 0;
  if (lam == mx && c == 1) return cout << "0", 0;
  sort(cand.begin(), cand.end());
  while (lam <= cand[n - 1]) {
    lam += 1;
    cand[n - 1] -= 1;
    sort(cand.begin(), cand.end());
  }
  cout << lam - x;
}
