def f(li):
    if len(set(li)) != 3:
        return False
    if (li[0] + li[1]) % li[2] != 0:
        return False
    if (li[1] + li[2]) % li[0] != 0:
        return False
    if (li[2] + li[0]) % li[1] != 0:
        return False
    return True
def solve():
    li = sorted(map(int, input().split()))
    ans = 1e18
    c0 = li[1] - li[0]
    if c0 > 0 and f(li + [c0]):
        ans = c0
    c1 = li[0] + li[1]
    if f(li + [c1]):
        ans = min(ans, c1)
    return -1 if ans == 1e18 else ans
print(solve())