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