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 i, a, b, m, n;

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

    i = 0;
    while(1)
    {
        m = a - i;
        if(m<1)
            m = m * -1;

        n = b - i;
        if(n<1)
            n = n * -1;

        if(m == n)
        {
            printf("%d\n",i);
            printf("|%d-%d|=%d and |%d-%d|=%d so, %d satisfies the condition.",a,i,m,b,i,n,i);
            break;
        }
        i++;
    }

    return 0;
}