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