#include <bits/stdc++.h>
using namespace std;
int arr[200000];
int main() {
  int n, sum = 0;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  for (int i = 1; i < n; i++) {
    if (arr[i] >= arr[i - 1]) continue;
    sum += arr[i - 1] - arr[i];
    arr[i] = arr[i - 1];
  }
  cout << sum << endl;
}
