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>
#include <string.h>
#include <stdlib.h>
#include <math.h>

int main( int argc, char **argv )
{
    char    ans[256] = {};

    printf("Input number (ex: 2 16): ");
    if ( fgets( ans, sizeof(ans), stdin ) == NULL )
    {
        printf("Error: Invalid input data\n");
        exit(1);
    }

    int ibuf1=0, ibuf2=0;
    const int n = sscanf( ans, "%d %d", &ibuf1, &ibuf2 );
    if ( n != 2 )
    {
        printf("Error: Wrong number of arguments.\n");
        exit(2);
    }

    if ( ibuf1 == ibuf2 )
    {
        printf("Error: Equal number of  arguments.\n");
        exit(3);
    }

    int ibuf3 = 0, large_num = 0;
    if ( ibuf1 < ibuf2 )
    {
        ibuf3 = ibuf2 - ibuf1;
        large_num = ibuf2;
    } else {
        ibuf3 = ibuf1 - ibuf2;
        large_num = ibuf1;
    }

    if ( (ibuf3 % 2) != 0 )
    {
        printf("IMPOSSIBLE\n");
    } else {
        printf(" %d\n", large_num - (ibuf3 / 2) );
    }

    exit(0);
}