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()
{
    int a,b,d;
    scanf("%d %d",&a,&b);
    if(a != b){
            int f=0;
        if(a>b){
            for(d=1;d<=a;d++){
                if(abs(a-d) == (b-d)){
                    f=1;
                    break;
                }
            }
            if(f == 1){
            printf("%d",d);
            }
            else
                printf("IMPOSSIBLE");
        }
        else{
            for(d=1;d<=b;d++){
                if(abs(a-d) == (b-d)){
                    f=1;
                    break;
                }
            }
            if(f == 1){
            printf("%d",d);
            }
            else
                printf("IMPOSSIBLE");
        }
    }

}