#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  long long n;
  cin >> n;
  long long a[n];
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
  }
  long long wm = 0;
  int ans = 0;
  for (long long i = 0; i < n; i++) {
    if (a[i] < wm) {
      ans += wm - a[i];
      a[i] = wm;
    }
    wm = max(wm, a[i]);
  }
  cout << ans;
}
