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