#!/usr/bin/env PyPy3

from collections import Counter, defaultdict, deque
import itertools
import re
import math
from functools import reduce
import operator
import bisect
from heapq import *
import functools
mod=998244353

import sys
input=sys.stdin.readline

a=list(map(int,input().split()))
s = set(a)
if len(s) == 1:
    for i in range(4):
        if i in s:
            print(i)
elif len(s) == 2:
    print(a[0]^a[1]^a[2])
else:
    for i in range(4):
        if not i in s:
            print(i)