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;
    int btm, top;
    /* 入力 */
    scanf("%d%d", &A, &B);
    
    if(A <= B){
        btm = A;top = B;
    }else{
        btm = B;top = A;
    }
    /* 判定 */
    for(int i = btm; i <= top; i++){
        if( abs(A-i) == abs(B-i) && A != B ){
            printf("%d", i);
            return 0;
        }
    }
    printf("IMPOSSIBLE\n");
    return 0;
}