from collections import defaultdict
tmp = list(map(int,input().split()))
cnt = defaultdict(int,default=0)
numbers = set()
for i in tmp:
    numbers.add(i)
    cnt[i] += 1
if len(numbers) == 1:
    print(tmp[0])
elif len(numbers) == 2:
    for i in tmp:
        if cnt[i] == 1:
            print(i)
            exit()
else:
    for i in range(4):
        if i in numbers:
            continue
        print(i)
        exit()