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: Runtime Error)

#include<stdio.h>
int main(void)
{
	int i,N,p[100]={},count=0;
	scanf("%d",&N);
	for(i=0;i<N;i++){
		scanf("%d",&p[i]);
	}
	if(N!=2){
		for(i=0;i<N;i++){
			if(p[i]!=i){
				count++;
			}
		}
		if(count<=2){
			printf("YES\n");
		}else{
			printf("NO\n");
		}
	}if(N==2){
		printf("YES\n");
	}
	return 0;
}

//  2 6 3 9 4  ,  4 6 2 5 8 ,3 14 2