#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;
  for (int i = 1; i <= N - 1; i++) {
    int j = 0;
    while (true) {
      if (A[i - 1] <= A[i]) {
        sum += j;
        break;
      }
      j++;
      A[i]++;
    }
  }
  cout << sum << endl;
}
