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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <cstdio>
#include <algorithm>
#include <math.h>
#include <stdlib.h>
#define LL long long
using namespace std;
int main()
{
 LL A,B,k,counter;
  counter=0;
 scanf("%lld %lld",&A,&B);

 for(k=0;k<100000000;k++){
 	
   if(A-B==2*k||A+B==2*k){
   	
     counter++;
     break;
   
   }
  
 }
  
  if(counter!=0){
 
 	printf("%lld",k); 
 
  }else{
  	
    puts("IMPOSSIBLE");
  
  }
  
  
     return 0;
}