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