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: Wrong Answer)

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let v: Vec<&str> = input.trim().split(" ").collect();

    let mut count = 1;
    loop {
        let a = &v[0].parse::<i32>().unwrap() - count;
        let b = &v[1].parse::<i32>().unwrap() - count;

        let a_diff = a - count;
        let b_diff = b - count;

        let a_abs = a_diff.abs();
        let b_abs = b_diff.abs();


        if a_abs == b_abs {
            println!("{}", count);
            break;
        }

        if count >= a || count >= b {
            println!("IMPOSSIBLE");
            break;
        }

        count = count + 1;
    }
}