fruits = list(map(int, input().split()))
cnt = {0: 0, 1: 0, 2: 0, 3: 0}
for f in fruits:
    cnt[f] += 1
res = list(map(lambda x: x != 0, cnt.values()))
if sum(res) == 1:
    for k, c in cnt.items():
        if c == 3:
            print(k)
elif sum(res) == 2:
    for k, c in cnt.items():
        if c == 1:
            print(k)
else:
    for k, c in cnt.items():
        if c == 0:
            print(k)