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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"fmt"
	"io"
	"os"
)

func proc(r io.Reader, w io.Writer) {
	var a, b int
	fmt.Fscanf(r, "%d %d", &a, &b)

	sum := a + b
	if sum%2 == 0 {
		fmt.Fprintf(w, "%d", sum/2)
		return
	}

	fmt.Fprintf(w, "IMPOSSIBLE")
}

func main() {
	proc(os.Stdin, os.Stdout)
}