#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 sum = 0;
  for (int i = 1; i < n; i++) {
    int diff = a.at(i - 1) - a.at(i);
    sum += max(0, diff);
    a.at(i) += max(0, diff);
  }
  cout << sum << endl;
}
