#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;
    if (a < m)
      ans += m - a;
    else
      m = a;
  }
  cout << ans << endl;
}
