#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.at(i);
  }
  int diff;
  int ans = 0;
  for (int i = 0; i < n - 1; i++) {
    if (a.at(i) > a.at(i + 1)) {
      diff = a.at(i) - a.at(i + 1);
      ans += diff;
      a.at(i + 1) += diff;
    }
  }
  cout << ans << endl;
}
