#include <bits/stdc++.h>
using namespace std;
int a, b, c, n;
int main() {
  ios_base::sync_with_stdio(false);
  ;
  cin >> a >> b >> c >> n;
  if (a - c < 0 || b - c < 0) return cout << -1, 0;
  if (a + b - c >= n) return cout << -1, 0;
  cout << n - (a + b - c);
  return 0;
}
