#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 maxi = a[0], ans = 0;
  for (int i = 0; i < n; i++) {
    if (maxi <= a[i]) {
      maxi = a[i];
    } else {
      ans += maxi - a[i];
    }
  }
  cout << ans << endl;
}
