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 PHP7 (7.0.15) (judge verdict: Accepted)

<?php
// # get a integer
// fscanf(STDIN, "%d", $a);
// # get two integers separated with half-width break
// fscanf(STDIN, "%d %d", $b, $c);
// # get a string
// fscanf(STDIN, "%s", $s);
// # output
// echo ($a+$b+$c)." ".$s."\n";

// sort
fscanf(STDIN, "%d %d %d", $a, $b, $c);

$arr = array($a, $b, $c);

sort($arr);

if($arr[1]-$arr[0] === $arr[2]-$arr[1])
	print_r("YES");
else
	print_r("NO");

//