#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, mx = 0, ans = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    int a;
    cin >> a;
    if (a > mx)
      mx = a;
    else
      ans += mx - a;
  }
  cout << ans;
}
