#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 cnt = 0, ans = 0;
  for (int i = 0; i < n - 1; i++) {
    if (a[i] > a[i + 1]) cnt = a[i] - a[i + 1];
    a[i + 1] = a[i];
    ans += cnt;
  }
  cout << ans;
}
