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 12.16.1) (judge verdict: Accepted)

"use strict";
// inputに入力データ全体が入る
const main = (input) => {
  const [a, b, c] = input.trim().split(" ").map(Number);
  if ((a + c) / 2 === b) {
    return "YES";
  } else {
    return "NO";
  }
};

// ※この行以降は編集しない (標準入力から一度に読み込みMainを呼び出します)
console.log(main(require("fs").readFileSync("/dev/stdin", "utf8")));