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;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Collections;

static class Extensions
{
    public static int ToInt(this string s) => int.Parse(s);
    public static long ToLong(this string s) => long.Parse(s);
    public static char ToChar(this string s) => char.Parse(s);
    public static double ToDouble(this string s) => double.Parse(s);
    public static int[] ToIntArray(this string s) => s.Replace(',', ' ').Split(' ').Select(x => x.ToInt()).ToArray();
    public static long[] ToLongArray(this string s) => s.Replace(',', ' ').Split(' ').Select(x => x.ToLong()).ToArray();
    public static double[] ToDoubleArray(this string s) => s.Replace(',', ' ').Split(' ').Select(x => x.ToDouble()).ToArray();
    public static Queue<T> ToQueue<T>(this IEnumerable<T> seq) => new Queue<T>(seq);
    public static Stack<T> ToStack<T>(this IEnumerable<T> seq) => new Stack<T>(seq);
    public static HashSet<T> ToHashSet<T>(this IEnumerable<T> seq) => new HashSet<T>(seq);
}
static class MyMath
{
    static public long GCD(long a, long b) => (a < b) ? GCD(b, a) : ((b == 0) ? a : GCD(b, a % b));
    static public long LCM(long a, long b) => (a > b) ? (a / GCD(a, b) * b) : (b / GCD(a, b) * a);
    static private readonly long _m = 1000000000 + 7;
    static public long Mod(long val) => (val % _m + _m) % _m;
    static private long extGCD(long a, long b, ref long x, ref long y) => (b == 0) ? (a + (x = 1) * 0 + (y = 0) * 0) : (extGCD(b, a % b, ref y, ref x) + (y -= a / b * x) * 0);
    static public long ModInverse(long a, long dummy1 = 0, long dummy2 = 0) => extGCD(a, _m, ref dummy1, ref dummy2) * 0 + Mod(dummy1);    
}

class Program
{
    static string[] ReadLines(int n) => Enumerable.Range(0, n).Select(s => Console.ReadLine()).ToArray();

    static void Main()
    {
        var ab = Console.ReadLine().ToIntArray();
        long A = ab[0];
        long B = ab[1];

        if ((A + B) % 2 == 0)
        {
            Console.WriteLine((A + B) / 2);
            return;
        }

        Console.WriteLine("IMPOSSIBLE");
    }
    
}