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"

func main() {
	var a, b int
	fmt.Scan(&a, &b)

	for i := 1; i < 1000000000; i++ {
		if a-i == -(b-i) {
			fmt.Println(i)
			return
		} else if  -(a-i) == b-i {
			fmt.Println(i)
		}
	}
	fmt.Println("IMPOSSIBLE")


}