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: cn = int(input())
  enm = input().split(" ")
  brv = input().split(" ")
  taoshita = int(0)
  
  for i in range(0, cn):
      taoshita = taoshita + min(abs(int(brv[i])), abs(int(enm[i])))
      enm[i] = int(enm[i]) - int(brv[i])
      if int(enm[i]) < 0:
          brv[i] = abs(int(enm[i]))
          enm[i] = 0
          if (int(enm[i + 1]) - int(brv[i])) < 0:
              taoshita = taoshita + int(enm[i + 1])
              enm[i + 1] = 0
          if (int(enm[i + 1]) - int(brv[i])) >= 0:
              enm[i + 1] = int(enm[i + 1]) - int(brv[i])
              taoshita = taoshita + int(brv[i])
  print(taoshita)
  
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"}]