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