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# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
class Program
{
  static void Main()
  {
    string[] input = Console.ReadLine().Split(' ');
    long A = long.Parse(input[0]);
    long B = long.Parse(input[1]);
    string ans;
    
    if ((A + B) % 2 == 1)
    {
      ans = "IMPOSSIBLE";
    }
    else
    {
      ans = ((A + B) / 2).ToString();
    }
    
    Console.WriteLine(ans);
  }
}