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