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