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)

/* ABC135-A.c
   Takatan */

#include<stdio.h>
#include<math.h>

int main(void){
    int number1, number2;
    double answer;

    scanf( "%d %d" , &number1, &number2);
    answer = ((double)(number1+number2) )/ 2;
    if(floor(answer)==ceil(answer)){
        printf( "%1.f\n" , answer);
    }else{
        printf( "IMPOSSIBLE\n" );
    }
    return 0;
}