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