#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, sum = 0;
  int a[n + 5];
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i <= n - 1; i++)
    if (a[i] > a[i + 1]) {
      sum = sum + a[i] - a[i + 1];
      a[i + 1] = a[i];
    }
  cout << sum << endl;
  return 0;
}
