from collections import Counter

lst = [int(i) for i in input().strip().split(' ')]

c = Counter(lst)

if len(c) == 3:
  for i in range(4):
    if not(i in lst):
      print(i)
      exit(0)
elif len(c) == 2:
  cc = [i[0] for i in c.items() if i[1] == 1][0]
  print(cc)
else:
  cc = list(c.keys())
  print(cc[0])