v = [int(x) for x in input().split()]
vs = set(v)
if len(vs) == 1:
    print(v.pop())
elif len(vs) == 3:
    print(({0, 1, 2, 3} - vs).pop())
else:
    x = vs.pop()
    if v.count(x) == 1:
        print(x)
    else:
        print(vs.pop())