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)

// ABC135 A
//20190908
using System;
using System.Linq;
using static System.Math;

namespace A
{
    class Program
    {
        static void Main(string[] args)
        {
            // Console.WriteLine("Hello World!");
            var INPUT = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int A = INPUT[0];
            int B = INPUT[1];

            int ansA = Abs(A - B) - 1;
            int ansB = Abs(B - A) - 1;

            if (ansA % 2 == 0 && ansB % 2 == 0)
            {
                Console.WriteLine("IMPOSSIBLE");
            }
            else
            {
                int K = Min(A,B) + (ansA / 2) + 1;
                Console.WriteLine(K);
            }

        }
    }
}