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: Accepted)

#include<stdio.h>
 int zetta(int x){
	 if(x<0)
		 return -x;
	 else
		 return x;
 }
 
	 int main(){
		 int a,b;
		 int c,k;
		 scanf("%d%d",&a,&b);
		 c=a+b;
		 k=zetta(c)/2;
		 if(c%2==1)
			 printf("IMPOSSIBLE");
		 else
			 printf("%d",k);
		 return 0;
	 }