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