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)
{
        unsigned long long A,B,K=0,S1=0,S2=0;
        printf("A = ");scanf("%d",&A);
        printf("B = ");scanf("%d",&B);

        K = (A*A - B*B) / (2*A - 2*B);
        if((A*A - B*B) % (2*A - 2*B) != 0)
         {
                printf("IMPOSIBLE\n");
                }
        else printf("%d\n",K);
        return 0;
        }