#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 = 0;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] < mx) {
      ans += mx - a[i];
    }
    mx = max(a[i], mx);
  }
  cout << ans << endl;
}
