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(dead_code)]
fn scan<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

#[allow(dead_code)]
fn scan_split<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split_whitespace()
        .map(|t| t.parse().ok().unwrap()).collect()
}

#[allow(dead_code)]
fn scan_split_with<T: std::str::FromStr>(ch: char) -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split(ch)
        .map(|t| t.parse().ok().unwrap()).collect()
}

#[allow(dead_code)]
fn scan_lines<T: std::str::FromStr>(n: usize) -> Vec<T> {
    (0..n).map(|_| scan::<T>()).collect()
}

#[allow(unused_macros)]
macro_rules! scan_as_tuple {
    ( $t:ty ) => {
        (scan::<$t>(),)
    };
    ( $( $t:ty ),* ) => {
        {
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).ok();
            let v = s.trim().split_whitespace();
            let mut it = v;
            let ret = ( $(it.next().unwrap().parse::<$t>().ok().unwrap()),* );
            assert!(it.next() == None, "not all tokens consumed");
            ret
        }
    };
}

fn main() {
    let (a, b, c) = scan_as_tuple![i32, i32, i32];
    println!("{}", if b - a == c - b { "YES" } else { "NO" });
}