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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

int main(){
  
  int A,B;
  cin >> A >> B;
  
  bool suc = 0;
  
  int K = (A - B);
  if (K % 2 == 0 && K >= 0){
    cout << K / 2<< endl;
    suc = 1;
  }
  
  if (suc < 1){
    K = (A + B);
    if (K % 2 == 0 && K >= 0){
      cout << K / 2 << endl;
      suc = 1;
    }
  }
  
  if (suc < 1){
    cout << "impossible" << endl;
  }
}