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