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 read_vec<T: std::str::FromStr>() -> Vec<T> {
  let mut s = String::new();
  std::io::stdin().read_line(&mut s).ok();
  s.trim().split_whitespace()
    .map(|e| e.parse().ok().unwrap()).collect()
}

fn main() {
  let ab: Vec<f64> = read_vec();
  let average = (ab[0] + ab[1]) / 2.0;
  if average % 1.0 != 0.0 { println!("IMPOSSIBLE"); } else { println!("{}", average as i32) };
}