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 9.2.1) (judge verdict: Wrong Answer)

#include<stdio.h>
#include<math.h>
#include<stdlib.h>
int main()
{
    int A,B,K,res1,res2;
    scanf("%d %d",&A, &B);
    K = (A+B)/2;
    res1 = abs(A-K);
    res2 = abs(B-K);
    if(abs(A-K) == abs(B-K)){
        printf("\n%d\n",K);
        printf("|%d-%d| = %d and |%d-%d| = %d, so %d satisfies the condition.\n",A,K, res1,B,K,res2,K);
    }else if(abs(A-K) != abs(B-K)){
        printf("IMPOSSIBLE\nNo integer satisfies the condition.\n");
    }else
    exit(0);
    return 0;
}