from collections import Counter

L = list(map(int, input().split()))

L_s = set(L[0:3])

ans = {0, 1, 2, 3}

if(len(L_s) == 3):
    output = list(ans - L_s)[0]
elif(len(L_s) == 2):
    C = Counter(L)
    for k, v in C.items():
        if(v == 1):
            output = k
else:
    output = L[0]

print(output)