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(void)
{
    int a,b;
    int ans;
    int result;

    scanf("%d %d", &a, &b);

    if (a > b)
    {
        ans = a - b;
    }

    else
    {
        ans = b - a;
    }
    
    

    if (ans % 2 == 0 && ans < a){
       result = a - (ans / 2);
    }

    else if(ans % 2 == 0 && ans > a){
        result = a + (ans / 2);
    }
    

    else {
        printf("INPOSSIBLE\n");
        return 0;
    }

    if (result < 0)
    {
        printf("%d\n", result * -1);
    }

    else
    {
        printf("%d\n", result);
    }
    
    
    return 0;

}