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)
  mod = 10**9 + 7
  dp = [[0] * 13 for i in range(l)]
  rem = [1, 10, 9, 12, 3, 4]
  for i in range(l)[::-1]:
      dig = i % 6
      if s[i] == "?":
          if i == l - 1:
              for j in range(10):
                  dp[-1][(j * rem[dig]) % 13] = 1
          else:
              for j in range(10):
                  for h in range(13):
                      dp[i][(h + j * rem[dig]) % 13] += dp[i + 1][h]
                      dp[i][(h + j * rem[dig]) % 13] %= mod
      else:
          a = int(s[i])
          if i == l - 1:
              dp[i][(a * rem[dig]) % 13] = 1
          else:
              for j in range(13):
                  dp[i][(j + a * rem[dig]) % 13] += dp[i + 1][j]
                  dp[i][(j + a * rem[dig]) % 13] %= mod
  print(dp[0][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"}]