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<string.h>
#include<stdlib.h>
#include<math.h>
long long cmpfunc (const void * a, const void * b) {
   return ( *(long long*)a - *(long long*)b );
}



int main(void){
   
long long int test,i,j,n,count,flag=0,o1=0,o2=0,b1,x,m,l,max,sum2,min,f,c,r,o,sum1,sum=0,y,a,b,count1=0,a2,b2;

scanf("%lld%lld",&a,&b);
if(a>=b && b!=0){
  
  
         printf("%lld",a-abs(a-b)/2);
  
}else{
     
   if(b>=a && a!=0){
   
       printf("%lld",b-abs(a-b)/2);
    }else{
        printf("IMPOSSIBLE");
    }

}
return 0;
}