text: from collections import Counter
  
  
  class Solution:
      def findTheDifference(self, s, t):
  
          s = Counter(s)
          t = Counter(t)
  
          for key in t.keys():
              if s[key] != t[key]:
                  return key
  
  
  
  if __name__ == '__main__':
  
  	test_case = Solution()
  	s, t = [input() for _ in range(2)]
  	print('\n' + test_case.findTheDifference(s, t))