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