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: # region header
  import sys
  import math
  from bisect import bisect_left, bisect_right, insort_left, insort_right
  from collections import defaultdict, deque, Counter
  from copy import deepcopy
  from fractions import gcd
  from functools import lru_cache, reduce
  from heapq import heappop, heappush
  from itertools import (
      accumulate,
      groupby,
      product,
      permutations,
      combinations,
      combinations_with_replacement,
  )
  from math import ceil, floor, factorial, log, sqrt, sin, cos
  from operator import itemgetter
  from string import ascii_lowercase, ascii_uppercase, digits
  
  sys.setrecursionlimit(10**6)
  INF = float("inf")
  MOD = 10**9 + 7
  
  
  def rs():
      return sys.stdin.readline().rstrip()
  
  
  def ri():
      return int(rs())
  
  
  def rf():
      return float(rs())
  
  
  def rs_():
      return [_ for _ in rs().split()]
  
  
  def ri_():
      return [int(_) for _ in rs().split()]
  
  
  def rf_():
      return [float(_) for _ in rs().split()]
  
  
  def divisors(n, sortedresult=True):
      div = []
      for i in range(1, int(n**0.5) + 1):
          if n % i == 0:
              div.append(i)
              if i != n // i:
                  div.append(n // i)
      if sortedresult:
          div.sort()
      return div
  
  
  # endregion
  a, b, c = ri_()
  print("YES" if b - a == c - b else "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."}]