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>

void swap (int *a,int *b);

int main (void){
  int a,b,k;
  int res=0;

  int i;
  if(scanf("%d %d",&a,&b) == 2){
    if ( b < a) swap(&a,&b);
    for(k=0; k<=b; k++){
      if ( abs(a-k) == abs(b-k) ) {
	printf("%d\n",k);
	return 1;
      }
    }
    printf("IMPOSSIBLE\n");
  }
  return 0;
  
}

void swap (int *a,int *b){
  int temp ;
  temp = *a;
  *a = *b;
  *b = temp;

  return;
}