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: Runtime Error)

using System;
class Program
    {
        static void Main(string[] args)
        {
            string[] input1 = Console.ReadLine().Split(' '); 
            Console.WriteLine((Sum(int.Parse(input1[0])) * Sum(int.Parse(input1[1]))) % MOD);
        }     
        private const int MOD = 1000000007;
        static long Sum(int n){
            string[] input3 = Console.ReadLine().Split(' ');
            long Sum = 0;
            for (int i = 0; i < n; i++) {
                 Sum += long.Parse(input3[i]) * (2 * i - n + 1); 
            }
            return Sum % MOD;
         }                        
    }