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 Rust (1.15.1) (judge verdict: Accepted)

use std::io;

fn main() {
    let ab: Vec<i8> = read_input_vec();
    if ab[1] - ab[0] == ab[2] - ab[1] {
        println!("YES");
    } else {
        println!("NO");
    }
}

fn read_input<T: std::str::FromStr>() -> T {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    return input.parse::<T>().ok().unwrap();
}

fn read_input_vec<T: std::str::FromStr>() -> Vec<T> {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    return input
        .split_whitespace()
        .map(|x| x.parse::<T>().ok().unwrap())
        .collect();
}