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.Text;

namespace codeTester.contest190727
{
    class ProblemA
    {
        static List<long> ab = new List<long>();
        static string answer = "";

        public static void Main(string[] args)
        {
            Input();
            Logic();
            Output();
        }

        static void Input()
        {
            ab = ReadSpaceSeparatedValues();
        }

        public static List<long> ReadSpaceSeparatedValues()
        {
            List<long> values = new List<long>();
            string[] inputs = Console.ReadLine().Split(' ');
            foreach (string input in inputs)
            {
                values.Add(long.Parse(input));
            }

            return values;
        }

        static void Logic()
        {
            long a = ab[0];
            long b = ab[1];
            if ((a + b) % 2 == 0) {
                answer = ((a + b) / 2).ToString();
                return;
            }
            answer = "IMPOSSIBLE";
        }

        static void Output()
        {
            Console.WriteLine(answer);
        }
    }
}