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 C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

int main(void) {

	unsigned long A, B, K, tmp;

	scanf("%lu%lu", &A, &B);

	//Aを大きい値に設定
	if(B > A){
		tmp = A;
		A = B;
		B = tmp;
	}

	if((A-B)%2 != 0){
		printf("IMPOSSIBLE");
	}else{
		K =(A - (A -B)/2) ;
		printf("%lu", K);
	}


	return EXIT_SUCCESS;
}