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 (Clang 10.0.0) (judge verdict: Accepted)

#include<stdio.h>
int main()
{
    int A,B,i,c,d,e=0;
    long long int K;
    scanf("%d%d",&A,&B);
    if(A>B)
    {
        for(i=1;i<A;i++)
            {
                K=i;
                c=A-K;
                d=B-K;
                if(c<0)
                {
                    c=c*(-1);
                }
                if(d<0)
                {
                    d=d*(-1);
                }
                if(c==d)
                {
                   e++;
                    break;
                }
            }
    }
    else if(A<B)
    {
        for(i=1;i<B;i++)
            {
                K=i;
                c=A-K;
                d=B-K;
                if(c<0)
                {
                    c=c*(-1);
                }
                if(d<0)
                {
                    d=d*(-1);
                }
                if(c==d)
                {
                    e++;
                    break;
                }
            }
    }
    if(e>0)
    {
        printf("%lld\n",K);
    }
    else
    {
        printf("IMPOSSIBLE\n");
    }

}