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: Runtime Error)

using System;

class Program
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());

        string[] input = Console.ReadLine().Split(' ');
        int[] b = new int[n];
        int[] c = new int[n];
        int i = 0;
        foreach (string s in input)
        {
            b[i] = int.Parse(s);
            c[i] = b[i];
            i++;
        }
        for (int ii = 0; ii < n - 1; ii++)
        {
            for (int jj = ii+1; jj < n; jj++)
            {
                if (b[ii] > b[jj])
                {
                    int buf = b[ii];
                    b[ii] = b[jj];
                    b[jj] = buf;
                }
            }

        }
        int counter = 0;
        for (int ii = 0; ii < n; ii++)
        {
            if (b[ii] != c[ii])
            {
                counter++;
            }
            if (counter > 2)
            {
                Console.WriteLine("NO");
                return;
            }
        }




        if (counter == 2)
        {
            Console.WriteLine("YES");
        }
        else if(counter == 0)
        {
            for (int ii = 0; ii < n - 1; ii++)
            {
                for (int jj = ii+1; jj < n; jj++)
                {
                    if (b[ii] == b[jj])
                    {
                        Console.WriteLine("YES");
                        return;
                    }
                }

            }
            Console.WriteLine("NO");
        }
        else
        {
            Console.WriteLine("NO");
        }

    }
}