ls = sorted(list(map(int,input().split())))
bools = [false for i in range(4)]
for i in ls:
	bools[i] = true
lg = bools.count(true)
if lg==1:
	print(ls[0])
elif lg==2:
	if ls[0]==ls[1]:
		print(ls[2])
	else:
		print(ls[0])
else:
	print(bools.index(false))