#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; ++i) cin >> A[i];
  int sum = 0;
  int j = 0;
  for (int i = 1; i <= N - 1; i++) {
    while (true) {
      if (A[i - 1] - (A[i] + j) <= 0) {
        sum += j;
        A[i] += j;
        break;
      } else
        j++;
    }
  }
  cout << sum << endl;
}
