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)

fn main() {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).expect("error!");
    let mut iter = line.split_whitespace();

    let a: i32 = iter.next().expect("error!")
        .parse().expect("error!");

    let b: i32 = iter.next().expect("error!")
        .parse().expect("error!");

    if a < b && (b - a) & 1 == 0 {
        let d = (b - a) / 2;
        println!("{}", a + d);
    } else if (a - b) & 1 == 0 { // a > b
        let d = (a - b) / 2;
        println!("{}", a - d);
    } else {
        println!("IMPOSSIBLE");
    }
}