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,B,J,K,L ;
  scanf("%d",&A) ;
  scanf("%d",&B) ;
  if(A==B){
    exit(1) ;
  }

  J=A+B ;
  if(J%2 == 1){
    printf("IMPOSSIBLE") ;
    exit(0) ;
    }
  K=(A-B)/2 ;
  L=(A+B)/2 ;
  if(A-K==B-K){
    printf("%d",K) ;
  }else{
    printf("%d",L) ;
  }
}