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: Accepted)

#include <stdio.h>
#include <stdlib.h>

int main (){
    int a=0;
    int b=0;
    int k=0;
    int stop=0;

    scanf("%d %d",&a,&b);

    if(a<b){
        stop=b;
    }else{
        stop=a;
    }

   for(int i=0;i<stop;i++){

       if(abs(a-k)==abs(b-k)){
           k=i;
           break;
       }else{
           k++;
       }
   }
    if(k<stop){
        printf("%d",k);
    }else{
        printf("IMPOSSIBLE");
    }

    return 0;
}