#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 operation = 0;
  for (int i = 0; i < n - 1; i++)
    if (arr[i] > arr[i + 1]) {
      int x = arr[i] - arr[i + 1];
      arr[i + 1] += x;
      operation += x;
    }
  cout << operation;
}
