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<math.h>
int main(void)
{
	int a,b;
	int flag = 1;
	scanf("%d %d",&a,&b);
	int i,j;
	if(a > b)
	{
		int temp = b;
		b = a;
		a = temp; 
	}
	for(i = a;i<=b;i++)
	{
		if(abs(a-i) == abs(b-i))
		{
			printf("%d",i);
			flag = 1;
			break;
		}
		else
			flag = 0;
	}
	if(flag == 0) printf("IMPOSSIBLE");
	return 0;
}