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)

/*
csc -debug A.cs && mono --debug A.exe <<< "2 16"
*/

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Reflection;

public class HelloWorld {
    public static void SolveCodeForces() {
        var A = cin.NextLong();
        var B = cin.NextLong();

        if (A % 2L != B % 2L) {
            System.Console.WriteLine("IMPOSSIBLE");
        }
        else {
            var K = (A + B) / 2L;
            System.Console.WriteLine(K);
        }
    }

    static Scanner cin = new Scanner();
    static StringBuilder cout = new StringBuilder();

    static public void Main () {
        SolveCodeForces();
        // IncreaseStack(SolveCodeForces);
        // System.Console.Write(cout.ToString());
    }

    public static void IncreaseStack(ThreadStart action, int stackSize = 128000000)
    {
        var thread = new Thread(action, stackSize);
        thread.Start();
        thread.Join();
    }
}

public static class Helpers {
    public static string Join<T>(this IEnumerable<T> arr, string sep = " ") {
        return string.Join(sep, arr);
    }

    public static void Swap<T>(ref T a, ref T b) {
        var tmp = a;
        a = b;
        b = tmp;
    }

    public static T[] CreateArray<T>(int length, Func<int, T> itemCreate) {
        var result = new T[length];
        for (var i = 0; i < result.Length; i++)
            result[i] = itemCreate(i);
        return result;
    }
}

public class Scanner
{
    private string[] line = new string[0];
    private int index = 0;

    public string Next() {
        if (line.Length <= index) {
            line = Console.ReadLine().Split(' ');
            index = 0;
        }
        var res = line[index];
        index++;
        return res;
    }

    public int NextInt() {
        return int.Parse(Next());
    }

    public long NextLong() {
        return long.Parse(Next());
    }

    public ulong NextUlong() {
        return ulong.Parse(Next());
    }

    public string[] Array() {
        line = Console.ReadLine().Split(' ');
        index = line.Length;
        return line;
    }

    public int[] IntArray(int emptyPrefixLen = 0) {
        var array = Array();
        var result = new int[emptyPrefixLen + array.Length];
        for (int i = 0; i < array.Length; i++)
            result[emptyPrefixLen + i] = int.Parse(array[i]);
        return result;
    }

    public long[] LongArray() {
        var array = Array();
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
            result[i] = long.Parse(array[i]);
        return result;
    }

    public ulong[] UlongArray() {
        var array = Array();
        var result = new ulong[array.Length];
        for (int i = 0; i < array.Length; i++)
            result[i] = ulong.Parse(array[i]);
        return result;
    }
}