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