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 greater1(int a, int b){
   if(a > b){
        return a;
    } 
    else if(b > a){
        return b;
    }
    else{
        return '=';
    }
}


int main(){
    int a, b, k, greater, smaller;
    scanf("%d %d", &a, &b);
    greater = greater1(a, b);
    if(greater == a){
        smaller = b;
    }
    else if(greater == b){
        smaller = a;
    }
    else{
        printf("0\n");
        return 0;
    }

    k = smaller;
    while(k != greater){
        if(abs(a - k) == abs(b - k)){
            printf("%d\n", k);
            return 0;
        }
        else{
            k++;
        }
    }

    printf("IMPOSSIBLE\n");
    return 0;
}