#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  long long a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  long long mx = -1;
  int cnt = 0;
  for (int i = 0; i < n; i++) {
    mx = max(mx, a[i]);
    if (a[i] < mx) {
      cnt += mx - a[i];
    }
  }
  cout << cnt;
}
