#include <bits/stdc++.h>
using namespace std;
int main() {
  int a;
  cin >> a;
  int max = 0;
  vector<int> b;
  int x;
  for (int i = 0; i < a; i++) {
    cin >> x;
    b.push_back(x);
  }
  sort(b.begin() + 1, b.end());
  int count = 0;
  if (b[0] > b[b.size() - 1]) {
    cout << "0";
  } else {
    while (1) {
      b[b.size() - 1]--;
      count++;
      b[0] = b[0] + 1;
      sort(b.begin() + 1, b.end());
      if (b[0] > b[b.size() - 1]) {
        cout << count;
        break;
      }
    }
  }
  printf("\n");
  return 0;
}
