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;
class Program
    {
        static void Main(string[] args)
        {
            string[] s = Console.ReadLine().Split(' ');
            bool b = new Calu().calu(s);
            if(b){
                Console.WriteLine("YES");
            }else{
                Console.WriteLine("NO");
            }
        }

        public class Calu {
            public Calu() { }
            public bool calu(string[] a){
                int b = int.Parse(a[0]);
                int c = int.Parse(a[1]);
                int d = int.Parse(a[2]);
                if (c - b == d - c) return true;
                return false;      
            }
        } 
    }