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::cmp::{max, min};

fn main() {
  let (a, b) = {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
    let mut ws = s.split_whitespace();
    let a: i64 = ws.next().unwrap().parse().unwrap();
    let b: i64 = ws.next().unwrap().parse().unwrap();
    (a, b)
  };

  let diff = max(a - b, b - a);
  if diff % 2 == 1 {
    print!("IMPOSSIBLE")
  } else {
    print!("{}", diff / 2 + min(a, b));
  }
}