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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>

int main(void){
  unsigned long n1, n2;
  scanf("%lu %lu",&n1,&n2);
  
  for(unsigned long i = 0; i <= n1 && i <= n2; i++){
  	 n1 = (n1-i) > 0?n1-i:-n1+i;
     n2 = (n2-i) > 0?n2-i:-n2+i;
    
    if(n1 == n2){
     	printf("%lu",i);
      	return 0;
    }
  }
  
  printf("IMPOSSIBLE");
	return 0; 
}