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

dict = {}
tmp = [0]*4

for i,v in enumerate(a):
    if v not in dict:
        dict[v] = 1
    else:
        dict[v]+=1

    tmp[v]+=1

if len(dict) == 1:
    print(a[0])
elif len(dict) == 2:
    for i,v in enumerate(tmp):
        if v == 1:
            print(i)
else:
    for i,v in enumerate(tmp):
        if v == 0:
            print(i)