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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
// using System.Collections.Generic;
// using System.Globalization;
// using System.IO;
// using System.Linq;
// using System.Text;
// using System.Text.RegularExpressions

class Program
{
    static void Main()
    {
        string[] arr = Console.ReadLine().Split();
        int A, B;
        A = int.Parse(arr[0]);
        B = int.Parse(arr[1]);
        Console.WriteLine((A + B) % 2 == 0 ? ((A + B) / 2).ToString() : "IMPOSSIBLE");
        return;
    }
}