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)

#![allow(unused_variables)]
#![allow(unused_imports)]
#![allow(dead_code)]
use std::io;
use std::str::FromStr;
use std::iter::FromIterator;
use std::cmp::{max, min, Ordering};
use std::collections::*;
use std::mem::{replace, swap};

// [NOTE] つかうときには trim() 入れないと RE になるので注意
// abcde
fn read_as_line() -> String {
    let mut line = String::new();
    io::stdin().read_line(&mut line).unwrap();
    line
}

// -3.2
fn read_as_primitive<T: FromStr>() -> T {
    let s = read_as_line();
    s.trim().parse().ok().expect("failed to parse input")
}

// 1 -3 0 999
fn read_as_collection<U: FromStr, T: FromIterator<U>>() -> T {
    let s = read_as_line();
    s.trim().split_whitespace().map(|s| s.parse().ok().expect("failed to parse")).collect()
}

// #.#.###..
fn read_as_chars<T: FromIterator<char>>() -> T {
    let s = read_as_line();
    s.trim().chars().collect()
}

// 1 abc
fn read_as_pair<T: FromStr, U: FromStr>() -> (T, U) {
    let s = read_as_line();
    let mut it = s.trim().split_whitespace();
    (
        it.next().unwrap().parse().ok().expect("failed to parse"),
        it.next().unwrap().parse().ok().expect("failed to parse"),
    )
}

// 1.5 -2 abc
fn read_as_triple<T: FromStr, U: FromStr, V: FromStr>() -> (T, U, V) {
    let s = read_as_line();
    let mut it = s.trim().split_whitespace();
    (
        it.next().unwrap().parse().ok().expect("failed to parse"),
        it.next().unwrap().parse().ok().expect("failed to parse"),
        it.next().unwrap().parse().ok().expect("failed to parse"),
    )
}

// Sorting ParitialOrd primitives like f64
// let mut v: Vec<f64> = vec![0.1, 1.0, 2.0];
// v.sort_by_key(|&x| Total(x));
// 
// src: https://qiita.com/hatoo@github/items/fa14ad36a1b568d14f3e
//      https://github.com/rust-lang/rust/pull/53938
//
#[derive(PartialEq, PartialOrd)]
pub struct Total<T>(pub T);

impl<T: PartialEq> Eq for Total<T> {}

impl<T: PartialOrd> Ord for Total<T> {
    fn cmp(&self, other: &Total<T>) -> std::cmp::Ordering {
        self.0.partial_cmp(&other.0).unwrap()
    }
}


fn main() {

    let (a, b): (usize, usize) = read_as_pair();
    if (a + b) % 2 == 1 {
        println!("IMPOSSIBLE")
    } else {
        println!("{}", (a + b) / 2);
    }
}