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 JavaScript (node.js v5.12) (judge verdict: Accepted)

"use strict";
const Main = (input) => {
	let xs = input.trim().split(' ').map(v=>parseInt(v,10));
    let A = xs[1] - xs[0];
    let B = xs[2] - xs[1];
	//console.log(A,B);
    if(A == B){
    	console.log('YES');
    }else{
    	console.log('NO')
    }
}
Main(require('fs').readFileSync('/dev/stdin','utf8'));