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)

#[allow(unused_imports)]
use std::io;
#[allow(unused_imports)]
use std::cmp;

#[allow(dead_code)]
fn getline() -> Vec<String> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok().unwrap();
    s.split(' ').map(|x| x.trim().to_string()).collect::<Vec<String>>()
}

#[allow(dead_code)]
fn get<T: std::str::FromStr>(s: &String) -> T {
    s.parse().ok().unwrap()
}

fn main() {
    let abc = getline();
    let a: i32 = get(&abc[0]);
    let b: i32 = get(&abc[1]);
    let c: i32 = get(&abc[2]);
    if c - b == b - a {
        println!("YES");
    } else {
        println!("NO");
    }
}