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