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: Accepted)

#include<iostream>
#include<math.h>
//#include<map>                                                                                                        
using namespace std;

int main()
{
  // 入力                                                                                                              
  int A, B;
  cin >> A >> B;

  double ans;

  ans = (double)(A + B) / 2;

  // ansが整数でない場合、答えは存在しない                                                               
  if((ans - (double)floor(ans)) > 0.0) cout << "IMPOSSIBLE" << endl;
  else cout << (int)ans << endl;

  return 0;
}