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: a = int(input())
  b = input().split(" ")
  sequence = []
  for i in range(0, a):
      sequence.append(int(b[i]))
  error = []
  for q in range(0, a - 1):
      if sequence[q] - sequence[q + 1] > 0:
          error.append(q)
          error.append(q + 1)
  
  if len(error) == 0:
      print("YES")
  
  elif len(error) == 2:
      if a == 2:
          print("YES")
      elif error[0] == 0:
          if sequence[error[0] + 2] - sequence[error[0]] > 0:
              print("YES")
          else:
              print("NO")
      elif error[1] == a:
          if sequence[error[1]] - sequence[error[1] - 2] > 0:
              print("YES")
          else:
              print("NO")
      else:
          if (
              sequence[error[1]] - sequence[error[1] - 2] > 0
              and sequence[error[0] + 2] - sequence[error[0]] > 0
          ):
              print("YES")
          else:
              print("NO")
  
  elif len(error) == 4:
      if error[1] == error[2]:
          if sequence[error[0]] > sequence[error[3]]:
              if error[0] == 0 and error[3] == a - 1:
                  if (
                      sequence[error[0]] - sequence[error[3] - 1] > 0
                      and sequence[error[0] + 1] - sequence[error[3]] > 0
                  ):
                      print("YES")
                  else:
                      print("NO")
              elif error[0] == 0:
                  if sequence[error[0] + 3] - sequence[error[0]] > 0:
                      print("YES")
                  else:
                      print("NO")
              elif error[1] == a:
                  if sequence[error[3]] - sequence[error[3] - 3] > 0:
                      print("YES")
                  else:
                      print("NO")
              else:
                  if (
                      sequence[error[3]] - sequence[error[3] - 3] > 0
                      and sequence[error[0] + 3] - sequence[error[0]] > 0
                  ):
                      print("YES")
                  else:
                      print("NO")
      elif (
          sequence[error[0]] - sequence[error[2]] > 0
          and sequence[error[1]] - sequence[error[3]] > 0
      ):
          if error[0] == 0 and error[3] == a - 1:
              if (
                  sequence[error[0]] - sequence[error[3] - 1] > 0
                  and sequence[error[0] + 1] - sequence[error[3]] > 0
              ):
                  print("YES")
              else:
                  print("NO")
          elif error[0] == 0:
              if sequence[error[3] + 1] - sequence[error[0]] > 0:
                  print("YES")
              else:
                  print("NO")
          elif error[3] == a:
              if sequence[error[3]] - sequence[error[0] - 1] > 0:
                  print("YES")
              else:
                  print("NO")
          elif (
              sequence[error[3]] - sequence[error[0] - 1] > 0
              and sequence[error[3] + 1] - sequence[error[0]] > 0
          ):
              print("YES")
          else:
              print("NO")
      else:
          print("NO")
  
  else:
      print("NO")
  
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"}]