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(void){

    int a,b,aa,bb;
    scanf("%d %d",&a,&b);
    int flag=0;
    for(int c=0;c<=1000000000;c++){
        aa=a-c;
        bb=b-c;
        if(abs(aa)==abs(bb)){
            printf("%d",c);
            flag=1;
        }
    }
    if(flag==0){
        printf("IMPOSSIBLE");
    }
    return 0;
}