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()
  s = s[::-1]
  mod = 10**9 + 7
  table = [[0] * 13 for _ in range(len(s) + 2)]
  table[0][0] = 1
  dd = 1
  for c in range(len(s)):
      if s[c] == "?":
          for n in range(10):
              for i in range(13):
                  table[c + 1][(i + n * dd) % 13] += table[c][i]
                  table[c + 1][(i + n * dd) % 13] %= mod
      else:
          num = int(s[c])
          for i in range(13):
              table[c + 1][(i + num * dd) % 13] += table[c][i]
              table[c + 1][(i + num * dd) % 13] %= mod
      dd = (dd * 10) % 13
  print(table[c + 1][5])
  
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"}]