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