l = list(map(int,input().split()))
n = [0]*4
for i in l:
    n[i] += 1
if len(set(l)) == 1:
    print(l[0])
elif len(set(l)) == 3:
    print(6-sum(l))
else:
    for i in range(4):
        if n[i] == 1:
            print(i)
            exit()