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;
namespace B_003_58
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            new Logic_A().Start();

        }

        public class Logic_A
        {
            public void Start()
            {
                var str = Console.ReadLine().Split(' ');
                var a = Convert.ToInt32(str[0]);
                var b = Convert.ToInt32(str[1]);
                var c = Convert.ToInt32(str[2]);

                Console.WriteLine(a + c == 2 * b ? "YES" : "NO");
            }
        }
    }
}