cf 991A

#include<stdio.h>

int main()
{
    int a, b, c, n;
    scanf("%d %d %d %d", &a, &b, &c, &n);

    if(a < c || b < c) printf("-1\n");
    else if(n > a + b - c) printf("%d\n", n - (a + b - c));
    else printf("-1\n");
}