#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 res = 0;
  int keep = a.at(0);
  for (int i = 1; i < n; i++) {
    if (a.at(i) < keep)
      res += keep - a.at(i);
    else
      keep = a.at(i);
  }
  cout << res << endl;
}
