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)

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main(void)
{
	int a, b,c,d;

	long x,y,z;

	scanf("%d%d", &a, &b);

	for (x = 0; x < 10000000000; x++) {
		if (a == 0 || b == 0) {
			printf("IMPOSSIBLE");
			break;
		}
		if (a % 2 != b % 2) {
			printf("IMPOSSIBLE");
			break;
		}
		y = a - x;
		z = b - x;
		if (y < 0)y = y * -1;
		if (z < 0)z = z * -1;
		if (y == z) {
			printf("%ld", x);
			break;
		}
		else if (x == 10000000000 && y != z) {
			printf("IMPOSSIBLE");
			break;
		}
	}

	return 0;
}