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)

/// 1行の文字を空白で分割してVec<T>にする。
fn read_split<T : std::str::FromStr>() -> Vec<T> {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input).unwrap();
    input.trim().split_whitespace().map(|x| x.parse().ok().unwrap()).collect()
}

fn main() {
    let mut input : Vec<i64> = read_split();
    input.sort();
    let small = input[0] % 2 == 0;
    let large = input[1] % 2 == 0;
    if small == large {
        
        println!("{}", input[1] - ((input[1] - input[0]) / 2));
    } else {
        println!("IMPOSSIBLE");
    }

}