output_description: Print `YES` if the arrangement of the poles is beautiful; print `NO`
  otherwise.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      a b c
attempt: a,b,c = map(input().split(), int)
  2.if a+c == 2*b:
  3.    print('YES')
  4.else:
  5.    print('NO')
  
problem_description: Statement
  
  Three poles stand evenly spaced along a line. Their heights are a, b and c
  meters, from left to right. We will call the arrangement of the poles
  _beautiful_ if the tops of the poles lie on the same line, that is, b-a = c-b.
  
  Determine whether the arrangement of the poles is beautiful.
samples: [{"input": "2 4 6", "output": "YES\n    \n\nSince 4-2 = 6-4, this arrangement of poles is beautiful.\n\n* * *"}, {"input": "2 5 6", "output": "NO\n    \n\nSince 5-2 \\neq 6-5, this arrangement of poles is not beautiful.\n\n* * *"}, {"input": "3 2 1", "output": "YES\n    \n\nSince 1-2 = 2-3, this arrangement of poles is beautiful."}]