Problem:
Score: 100 points

Problem Statement

We have two distinct integers A and B.

Print the integer K such that |A - K| = |B - K|.

If such an integer does not exist, print IMPOSSIBLE instead.

Constraints

All values in input are integers.

0 \leq A,\ B \leq 10^9

A and B are distinct.

Input

Input is given from Standard Input in the following format:

A B

Output

Print the integer K satisfying the condition.

If such an integer does not exist, print IMPOSSIBLE instead.

Sample Input 1

2 16

Sample Output 1

9

|2 - 9| = 7 and |16 - 9| = 7, so 9 satisfies the condition.

Sample Input 2

0 3

Sample Output 2

IMPOSSIBLE

No integer satisfies the condition.

Sample Input 3

998244353 99824435

Sample Output 3

549034394

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::io::Read;
fn main() {
    let mut buf = String::new();

    // 標準入力から全部bufに読み込む
    std::io::stdin().read_to_string(&mut buf).unwrap();

    // 読み込んだStringを空白で分解する
    let mut iter = buf.split_whitespace();

    let mut a: i32 = iter.next().unwrap().parse().unwrap();
    let mut b: i32 = iter.next().unwrap().parse().unwrap();
    if a < b {
        let t = a;
        a = b;
        b = t;
    }
    let d = a - b;
    if d % 2 == 0 {
        let v: i32 = d/2 as i32;
        let ans: i32 = v - a;
        println!("{}", -ans)
    } else {
        println!("IMPOSSIBLE")
    }

}