#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T lowbit(T n) {
  return (n ^ (n - 1)) & n;
}
template <class T>
inline int countbit(T n) {
  return (n == 0) ? 0 : (1 + countbit(n & (n - 1)));
}
template <class T>
T gcd(T a, T b) {
  return b == 0 ? a : gcd(b, a % b);
}
template <typename T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <class T>
T my_pow(T n, T p) {
  if (p == 0) return 1;
  T x = my_pow(n, p / 2);
  x = (x * x);
  if (p & 1) x = (x * n);
  return x;
}
template <class T>
T big_mod(T n, T p, T m) {
  if (p == 0) return (T)1;
  T x = big_mod(n, p / 2, m);
  x = (x * x) % m;
  if (p & 1) x = (x * n) % m;
  return x;
}
template <class T>
inline T Mod(T n, T m) {
  return (n % m + m) % m;
}
template <class T>
T multiplication(T n, T p, T m) {
  if (p == 0) return (T)0;
  T x = multiplication(n, p / 2, m);
  x = (x + x) % m;
  if (p & 1) x = (x + n) % m;
  return x;
}
bool isVowel(char ch) {
  ch = toupper(ch);
  if (ch == 'A' || ch == 'U' || ch == 'I' || ch == 'O' || ch == 'E')
    return true;
  return false;
}
template <class T>
T extract(string s, T ret) {
  stringstream ss(s);
  ss >> ret;
  return ret;
}
template <class T>
string itos(T n) {
  ostringstream ost;
  ost << n;
  ost.flush();
  return ost.str();
}
long long stoi(string s) {
  long long r = 0;
  istringstream sin(s);
  sin >> r;
  return r;
}
double toDouble(string s) {
  double r = 0;
  istringstream sin(s);
  sin >> r;
  return r;
}
int col[4] = {1, 0, -1, 0};
int row[4] = {0, 1, 0, -1};
struct node {
  long long x, y;
  long long z;
} arr[200005];
bool com(node a, node b) { return a.x < b.x; }
long long a, b, c = 0, n, m, r = 0, rr = 0, f = 0, ff = 0, ks = 0, res = 0;
long long ar[6543210], ar1[6543210];
string s, ss;
int main() {
  cin >> n;
  for (__typeof(n) i = 0; i < (n); i++) cin >> ar[i];
  long long res = ar[0];
  long long pos = 0;
  long long cnt = 0;
  for (int i = 1;; i++) {
    long long mx = 0;
    for (int j = 1; j < n; j++) {
      if (mx < ar[j]) {
        mx = ar[j];
        pos = j;
      }
    }
    if (res > mx) {
      break;
    }
    res++;
    ar[pos] = mx - 1;
    cnt++;
  }
  cout << cnt;
  return 0;
}
