#include <bits/stdc++.h>
using namespace std;
int main() {
  int arr[100001], n;
  long long ans = 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]) ans += arr[i - 1] - arr[i];
  cout << ans;
  return 0;
}
