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: Time Limit Exceeded)

#include<stdio.h>
int main()
{
    int a, b, c, d, k, found=0;
    scanf("%d %d", &a, &b);
    for(k=-1000000000; k<1000000000; k++)
    {

        c=a-k;
        if(c<0)
            c=c*(-1);
        d=b-k;
        if(d<0)
            d=d*(-1);

        if(c==d)
        {
            found=1;
            break;
        }

    }
    if(found)
        printf("%d\n", k);
    else
        printf("IMPOSSIBLE\n");
}