991A - If at first you don't succeed - CodeForces Solution

import sys, os, io
input = io.BytesIO(os.read(0, os.fstat(0).st_size)).readline

a, b, c, n = map(int, input().split())
if min(a, b) < c:
    ans = -1
else:
    x = n - (a + b - c)
    ans = x if x > 0 else -1
print(ans)