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.Linq;
using System.Numerics;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Diagnostics;
using System.Text.RegularExpressions;
using static System.Console;

namespace CsTest
{
    class Program
    {
        static void Main()
        {
            var input1 = AtCoder.ReadInt32Array();
            var a = input1[0];
            var b = input1[1];
            var c = input1[2];
            WriteLine((b - a == c - b) ? "YES" : "NO");
        }
    }

    public class AtCoder
    {
        public static string[] ReadStringArray() => ReadLine().Split(Array.Empty<char>(), StringSplitOptions.RemoveEmptyEntries);
        public static int[] ReadInt32Array() => ReadStringArray().Select(s => int.Parse(s)).ToArray();
        public static long[] ReadInt64Array() => ReadStringArray().Select(s => long.Parse(s)).ToArray();
    }
}