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: Accepted)

#include <stdio.h>
typedef long long ll;

int main(void){
  ll a, b;
  ll t;
  scanf("%lld %lld", &a, &b);

  // aを大きくする
  if(a<b){
    t=a;
    a=b;
    b=t;
  }

  // printf("%lld\n", a-b);
  if((a-b)%2==1){
    printf("IMPOSSIBLE\n");
  }else{
    printf("%lld\n", (a+b)/2);
  }

  return 0;
}