#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a;
  cin >> a;
  int b = a;
  long long ans = 0;
  for (int i = 2; i <= n; ++i) {
    int a;
    cin >> a;
    ans += max(0, b - a);
    b = a;
  }
  cout << ans << '\n';
  return 0;
}
