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::stdin;

fn main() {
    let mut ab = String::new();
    stdin().read_line(&mut ab).unwrap();
    let mut ab: Vec<isize> = ab.trim().split_whitespace().flat_map(str::parse).collect();
    ab.sort();
    let (a, b) = (ab[0], ab[1]);

    if (b - a) % 2 == 0 {
        println!("{}", a + (b - a) / 2);
    } else {
        println!("IMPOSSIBLE");
    }
}