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;
using System.Collections.Generic;
class Program
{
    static int x;
    static List<int> s = new List<int>();
    static string result;
    static void Main()
    {
        string[] tmp = Console.ReadLine().Split(' ');
        s.Add(int.Parse(tmp[0]));
        s.Add(int.Parse(tmp[1]));
        if ((s[0] + s[1]) % 2 == 0) { result = ((s[0] + s[1]) / 2).ToString(); } else { result = "IMPOSSIBLE"; }

        Console.WriteLine(result);
            Console.Out.Flush();
        
    }

 


}