#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> v(n);
  for (int i = 0; i < n; i++) cin >> v[i];
  int mx = v[0], stool = 0;
  for (int i = 1; i < n; i++) {
    if (mx > v[i])
      stool += (mx - v[i]);
    else if (mx < v[i])
      mx = v[i];
  }
  cout << stool << endl;
  return 0;
}
