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)

fn main() {
    let mode = "a";
    if mode == "a" {
        a::solve();
    }
}

mod a {
    use std;
    use std::io::Read;

    pub fn solve() {
        let mut buf = String::new();
        std::io::stdin().read_to_string(&mut buf).unwrap();

        let mut text = buf.split_whitespace();
        let a = text.next().unwrap().parse().unwrap();
        let b = text.next().unwrap().parse().unwrap();
        match calc(a, b) {
            None => println!("IMPOSSIBLE"),
            Some(x) => println!("{}", x),
        }
    }

    fn calc(a: i32, b: i32) -> Option<i32> {
        let total = a + b;
        if (total % 2) == 1 {
            return None;
        }
        let half = total / 2;
        if a > b {
            Some(a - half)
        } else {
            Some(a + half)
        }
    }
}