from collections import Counter
a = list(map(int,input().split()))
x = Counter(a)
if len(x) == 1:
    print(a[0])
elif len(x) == 2:
    for i in x:
        if x[i] == 1:
            print(i)
else:
    for i in range(4):
        if i not in a:
            print(i)