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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
 
int main(void) {
  int a,b;
  long k;
  
  scanf("%d %d",&a,&b);
  /*printf("%d %d\n",a,b);*/
  
  k=labs(a*a-b*b)%labs(2*a-2*b);
     
  /*printf("%d",k);*/
   if(k==0){
     k=abs(a*a-b*b)/abs(2*a-2*b);
     printf("%d",k);
   }
  else{
    printf("IMPOSSIBLE");
  }
   return 0;
}