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