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::io;

fn main() {
  let mut line = String::new();
  io::stdin().read_line(&mut line).expect("");
  let v: Vec<i32> = line.split(' ').map(|s| s.trim().parse().unwrap()).collect();
  let a = v[0] + v[1];
  if a % 2 == 0 {
    print!("{}\n", a / 2);
  } else {
    print!("IMPOSSIBLE\n");
  }
}