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,k,swap;
scanf("%d %d",&a,&b);
if (a > b)
{
  swap = a;
  a = b;
  b = swap;
}
if (a > 0)
{
    a = -a;
}
if (b < 0)
{
    b = -b;
}
for (k = 0;k < -a;k++)
{
    if (a+k == b-k)
    {
        break;
    }
}
if (a+k == b-k)
{
    printf ("%d",k);
}else{
    printf("IMPOSSIBLE");
}
return 0;
}