import sys
import collections
def resolve():
    tmp = str2d()
    adict = collections.defaultdict(int)
    bdict = collections.defaultdict(int)
    for color in tmp[1]:
        adict[color]+=1
    for color in tmp[2]:
        bdict[color]+=1

    for color in adict:
        if adict[color] != bdict[color]:
            print(color)
            return
        
def str1d():return sys.stdin.read().splitlines()
def str2d():return [list(s.split()) for s in str1d()]
resolve()