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::{BufRead, Read};

pub struct Scanner<R>
where
    R: BufRead,
{
    reader: R,
}

impl<R> Scanner<R>
where
    R: BufRead,
{
    pub fn new(reader: R) -> Self {
        Scanner { reader: reader }
    }

    pub fn next<T>(&mut self) -> T
    where
        T: ::std::str::FromStr,
        <T as ::std::str::FromStr>::Err: ::std::fmt::Debug,
    {
        self.reader
            .by_ref()
            .bytes()
            .map(|b| b.unwrap() as char)
            .skip_while(|c| c.is_whitespace())
            .take_while(|c| !c.is_whitespace())
            .collect::<String>()
            .parse::<T>()
            .unwrap()
    }
}

fn main() {
    let stdin = std::io::stdin();
    let stdin = stdin.lock();
    let mut sc = Scanner::new(stdin);

    let mut a: u64 = sc.next();
    let mut b: u64 = sc.next();
    if a > b {
        let tmp = a;
        a = b;
        b = tmp;
    }
    if (a + b) % 2 == 0 {
        println!("{}", (a + b) / 2);
    } else {
        println!("IMPOSSIBLE");
    }
}