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()
{
	long long int a, b, k = -1, temp1, temp2, min, max;
	scanf("%lld %lld", &a, &b);
	min = a;
	max = b;
	if (min > b)
	{
		min = b;
		max = a;
	}
	for (; min <= max; ++min)
	{
		temp1 = llabs(a-min);
		temp2 = llabs(b-min);
		if (temp1 == temp2)
		{
			k = min;
			break;
		}
	}
	if (k == -1)
	{
		printf("IMPOSSIBLE");
	} else {
		printf("%lld", k);
	}
	return 0;
}