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: N = int(input())
  N = N + 1
  monsters = list(map(int, input().split()))
  monsters.append(0)
  heros = list(map(int, input().split()))
  stack = 0
  count = 0
  point = 0
  life = 0
  for i in range(0, len(heros)):
      point = monsters[i] - (stack + heros[i])
      if point < 0:
          if abs(point) > monsters[i + 1]:
              stack = monsters[i + 1]
          else:
              stack = heros[i] - monsters[i]
          count += monsters[i]
      else:
          count += heros[i]
  count += stack
  print(count)
  
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"}]