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