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>
#include<stdlib.h>

main()
{
	int n=1,i,j,s=1;
	int *p,*q,*k=(int*)malloc(sizeof(int));
	while(n)
	{
		scanf("%d",&n);
		
		p=(int*)malloc(sizeof(int)*n);

		k=(int *)realloc(k,s*sizeof(int));
		for(i=0;i<n;i++)
		{
			scanf("%d",&p[i]);
			
		}
		for(i=0;i<n;i++)
		{
			for(j=0;j<n;j++)
			{
				if(p[j]==i+1 && j+1!=p[i])
				{             
				              k[s-1]=0;
				              break;
                }
             }
             if(k[s-1]==0)
                        break;
		}
		s++;
		free(p);
		}
	for(i=0;i<s-2;i++)
	{
		if(k[i]==0)
			printf("not ambiguous\n");
		else
			printf("ambiguous\n");
	}
	
	free(k);
	
}