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)

    macro_rules! input {
        (source = $s:expr, $($r:tt)*) => {
            let mut iter = $s.split_whitespace();
            input_inner!{iter, $($r)*}
        };
        ($($r:tt)*) => {
            let mut s = {
                use std::io::Read;
                let mut s = String::new();
                std::io::stdin().read_to_string(&mut s).unwrap();
                s
            };
            let mut iter = s.split_whitespace();
            input_inner!{iter, $($r)*}
        };
    }
     
    macro_rules! input_inner {
        ($iter:expr) => {};
        ($iter:expr, ) => {};
     
        ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
            let $var = read_value!($iter, $t);
            input_inner!{$iter $($r)*}
        };
    }
     
    macro_rules! read_value {
        ($iter:expr, ( $($t:tt),* )) => {
            ( $(read_value!($iter, $t)),* )
        };
     
        ($iter:expr, [ $t:tt ; $len:expr ]) => {
            (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
        };
     
        ($iter:expr, chars) => {
            read_value!($iter, String).chars().collect::<Vec<char>>()
        };
     
        ($iter:expr, usize1) => {
            read_value!($iter, usize) - 1
        };
     
        ($iter:expr, $t:ty) => {
            $iter.next().unwrap().parse::<$t>().expect("Parse error")
        };
    }
     
    fn main() {
    	input!{
        	a: u32,
            b: u32,
        }
		if (a + b) % 2 == 0 {
        	println!("{}", (a + b) / 2);
        } else {
			println!("{}", "IMPOSSIBLE");
        }
    }