def operation(a, b, c):
    res = condition1(a, b, c)
    if res >= 0:
        return res
    res = condition2(a, b, c)
    if res >= 0:
        return res
    res = condition3(a, b, c)
    if res >= 0:
        return res


def condition1(a, b, c):
    res = -1
    if a == b and a == c:
        res = a
    return res


def condition2(a, b, c):
    res = -1
    if a == b and a != c:
        res = c
    elif a == c and a != b:
        res = b
    elif b == c and a != b:
        res = a
    return res


def condition3(a, b, c):
    res = -1
    if a != b and a != c and b != c:
        res = abs(a + b + c - 6)

    return res


a, b, c = map(int, input().split())

print(operation(a, b, c))