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()
  dp = [[0] * 13 for _ in range(len(S))]
  mod = 10**9 + 7
  for i in range(13):
      if S[0] == "?":
          dp[0][i] = 1 if i <= 9 else 0
      else:
          dp[0][i] = 1 if int(S[0]) == i else 0
  for i in range(1, len(S)):
      if S[i] == "?":
          for j in range(10):
              for k in range(13):
                  dp[i][(k * 10 + j) % 13] = (
                      dp[i][(k * 10 + j) % 13] + dp[i - 1][k]
                  ) % mod
      else:
          j = int(S[i])
          for k in range(13):
              dp[i][(k * 10 + j) % 13] = (dp[i][(k * 10 + j) % 13] + dp[i - 1][k]) % mod
  print(dp[len(S) - 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"}]