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