#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n, ans = 0, c = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    int a;
    cin >> a;
    if (a < c)
      ans += (c - a);
    else
      c = a;
  }
  cout << ans << "\n";
}
