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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include<stdio.h>
typedef long long ll;



int main(void)
{
    int a,b;
    scanf("%d%d",&a,&b);
    int k;
    if((a*a - b*b)%(2*a - 2*b) == 0  )
    {
        k = (a*a - b*b)/(2*a - 2*b);
        printf("%d\n",k);
    }
    else
    {
        printf("IMPOSSIBLE\n");
    }

}