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;
use std::str::FromStr;
fn main() {
    let stdin = io::stdin();
    let mut s = String::new();
    stdin.read_line(&mut s).ok();
    s.pop();
    let mut abc : Vec<i32> = Vec::new();
    let it = s.trim().split_whitespace();
    for arg in it {
        abc.push(i32::from_str(&arg).expect("e"));
    }
    let a = abc[0];
    let b = abc[1];
    let c = abc[2];
    println!("{}", if b - a == c - b {"YES"} else {"NO"});
}