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: Time Limit Exceeded)

using System;
using System.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCoder
{
    public class AAddtion
    {
        public static void Main(string[] args)
        {
            new AAddtion().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            // 解答はココに書く
            var a = cin.ReadInt;
            var b = cin.ReadInt;
            var flg = true;
            for (int k = 0; k < Math.Abs(a - b); k++)
            {
                if (Math.Abs(a - k) == Math.Abs(b - k))
                {
                    WriteLine(k);
                    flg = false;
                    break;
                }
            }

            if (flg) WriteLine("IMPOSSIBLE");
        }
    }
}
public class ConsoleInput
{
    private readonly System.IO.TextReader _stream;
    private char _separator = ' ';
    private Queue<string> inputStream;
    public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
    {
        this._separator = separator;
        this._stream = stream;
        inputStream = new Queue<string>();
    }
    public string Read
    {
        get
        {
            if (inputStream.Count != 0) return inputStream.Dequeue();
            string[] tmp = _stream.ReadLine().Split(_separator);
            for (int i = 0; i < tmp.Length; ++i)
                inputStream.Enqueue(tmp[i]);
            return inputStream.Dequeue();
        }
    }
    public string ReadLine { get { return _stream.ReadLine(); } }
    public int ReadInt { get { return int.Parse(Read); } }
    public long ReadLong { get { return long.Parse(Read); } }
    public double ReadDouble { get { return double.Parse(Read); } }
    public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
    public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
    public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
}