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