Problem:
Score : 100 points

Problem 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.

Constraints

1 \leq a,b,c \leq 100

a, b and c are integers.

Input

Input is given from Standard Input in the following format:

a b c

Output

Print YES if the arrangement of the poles is beautiful; print NO otherwise.

Sample Input 1

2 4 6

Sample Output 1

YES

Since 4-2 = 6-4, this arrangement of poles is beautiful.

Sample Input 2

2 5 6

Sample Output 2

NO

Since 5-2 \neq 6-5, this arrangement of poles is not beautiful.

Sample Input 3

3 2 1

Sample Output 3

YES

Since 1-2 = 2-3, this arrangement of poles is beautiful.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Text;
using System.Linq;
using System.Collections.Generic;


class Program {

	const long mod = 1000000007;
	//public static int[] max;

	static void Main(string[] args) {

		char[] cs = new char[] { ' ', ',' };
		//int val = int.Parse(Console.ReadLine());
		int[] vals = parseAry(Console.ReadLine().Split(cs));
		//int val = int.Parse(Console.ReadLine());
		//string str = Console.ReadLine();
		//string[] strs = Console.ReadLine().Split(cs);

		//int[] vals = parseAry(Console.ReadLine().Split(cs));
		//int n = vals[0];
		//string res="";

		//int y = vals[0];
		//int x = vals[1];

		string re = "NO";
		if (vals[1]-vals[0]==(vals[2]-vals[1])) {
			re = "YES";
		}



		Console.WriteLine(re);


		return;
	}



	//　String[]  ----->  int[]
	static int[] parseAry(string[] str) {
		int[] nums = new int[str.Length];
		for (int i = 0; i < str.Length; i++) { nums[i] = int.Parse(str[i]); }
		return nums;
	}
	//　String[]  ----->  long[]
	static long[] parseAryl(string[] str) {
		long[] nums = new long[str.Length];
		for (int i = 0; i < str.Length; i++) { nums[i] = int.Parse(str[i]); }
		return nums;
	}



	//文字列を1文字ずつString[]に入れる
	static string[] strAry(String str) {
		string[] re = new string[str.Length];
		for (int i = 0; i < str.Length; i++) { re[i] = str[i].ToString(); }
		return re;
	}

}