new_contents: #!/usr/bin/py
  
  def lonelyinteger(a):
      answer = 0
      nums = {}
      for value in a:
          if value in nums:
              nums[value] += 1
          else:
              nums[value] = 1
      for value in a:
          if nums[value] < 2:
              answer = value
              break
      return answer
      
  a = input()
  b = map(int, raw_input().strip().split(" "))
  print lonelyinteger(b)
  
subject: Create solution3.py