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