#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, c = 0;
  cin >> n;
  int a[n + 1];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (int i = 0; i < n - 1; i++) {
    while (a[i] > a[i + 1]) {
      a[i + 1]++;
      c++;
    }
  }
  std::cout << c << std::endl;
}
