output_description: Print the integer K satisfying the condition.
  
  If such an integer does not exist, print `IMPOSSIBLE` instead.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      A B
attempt: S = input()
  l = len(S)
  nhatena = 0
  for s in S:
      if s != "?":
          break
      nhatena += 1
  
  min = 10 ** (l - 1 - nhatena)
  max = 10 ** (l) - 1
  
  # print(min)
  # print(max)
  
  mink = (
      int((min - 5) / 13) + 1
      if not ((min - 5) / 13.0).is_integer()
      else int((min - 5) / 13)
  )
  maxk = (
      int((max - 5) / 13) + 1
      if not ((max - 5) / 13.0).is_integer()
      else int((max - 5) / 13)
  )
  
  # print(mink)
  # print(maxk)
  
  cnt = 0
  for k in range(mink, maxk):
      v = 13 * k + 5
      val = ("%0" + str(l) + "d") % (v)
      # Sとの一致判定
      for i, s in enumerate(S):
          if s != "?":
              if s != val[i]:
                  break
      else:
          cnt += 1
  
      if cnt == 1000000007:
          cnt = 0
  
  print(cnt)
  
problem_description: Statement
  
  We have two distinct integers A and B.
  
  Print the integer K such that |A - K| = |B - K|.
  
  If such an integer does not exist, print `IMPOSSIBLE` instead.
samples: [{"input": "2 16", "output": "9\n    \n\n|2 - 9| = 7 and |16 - 9| = 7, so 9 satisfies the condition.\n\n* * *"}, {"input": "0 3", "output": "IMPOSSIBLE\n    \n\nNo integer satisfies the condition.\n\n* * *"}, {"input": "998244353 99824435", "output": "549034394"}]