#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int max = 0;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    int a;
    cin >> a;
    if (a < max) {
      ans += max - a;
    } else {
      max = a;
    }
  }
  cout << ans << endl;
  return 0;
}
