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