#include <bits/stdc++.h>
using namespace std;
bool isPrime(int x) {
  if (x == 1) return false;
  for (long long i = 2; i <= sqrt(x); i += 1)
    if (x % i == 0) return false;
  return true;
}
void print(int a[], int n) {
  for (long long i = 0; i < n; i += 1) cout << a[i] << " ";
}
void iarr(int *a, int n) {
  for (long long i = 0; i < n; i += 1) cin >> a[i];
}
long long power(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = ((res % 1000000007) * (a % 1000000007)) % 1000000007;
    a = ((a % 1000000007) * (a % 1000000007)) % 1000000007;
    b >>= 1;
  }
  return res;
}
long long numFactors(long long x) {
  long long cnt = 0;
  for (long long i = 1; i <= sqrt(x); i += 1) {
    if (x % i == 0) {
      if (x / i == i)
        cnt += 1;
      else
        cnt += 2;
    }
  }
  return cnt;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout << fixed;
  cout.precision(10);
  ;
  int n;
  cin >> n;
  int a[n];
  iarr(a, n);
  long long cnt = 0;
  while (true) {
    int ind = -1, mx = a[0];
    for (long long i = 1; i < n; i += 1) {
      if (a[i] >= mx) {
        ind = i;
        mx = a[i];
      }
    }
    if (ind != -1) {
      a[ind]--;
      a[0]++;
      cnt++;
    } else if (ind == -1) {
      break;
    }
  }
  cout << cnt;
  return 0;
}
