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>
//　10min
int main(void){
  int a, b;
  int ans = 0;

  scanf("%d%d", &a, &b);

  if((a-b)%2 == 0){
    ans = (a-b) / 2;
  }else if((b-a)%2 == 0){
    ans = (b-a) / 2;
  }else{
    printf("IMPOSSIBLE\n");
    return 0;
  };

  if(ans < 0){
    ans *= (-1);
  }
  printf("%d\n", ans);
  
  return 0;
}