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: Wrong Answer)

#include<stdio.h>
int main()
{
    int a,b,k;
    scanf("%d%d",&a,&b);
    if(b>a)
    for(int i=b;b>=a;i--)
    {
        if((i-a)==(b-i))
            {
                printf("%d",i);
                break;
            }
        else
        {
         printf("IMPOSSIBLE");
         break;
        }
    }
    else if(a>b)
    for(int i=a;a>=b;i--)
    {
        if((a-i)==(i-b))
        {
            printf("%d",i);
        }
        else
        {
         printf("IMPOSSIBLE");
         break;
        }
    }
    return 0;
}