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