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: Accepted)

#include <stdio.h>
int main(void)
{
    int a, b, c;
    scanf("%d%d", &a, &b);
    int i, max, min;
    if (a >= b)
    {
        max = a;
        min = b;
    }
    else
    {
        max = b;
        min = a;
    }
  //printf("%d%d",min,max);
    for (i = 0; i < max; i++)
    {
      //printf("%d   %d\n",max-i,min + i);
        if (max - i == min+i)
        {
            printf("%d", max-i);
            return 0;
        }
    }
    printf("IMPOSSIBLE");
    return 0;
}