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;

namespace ConsoleApp1
{
	class Program
	{
		static void Main(string[] args)
		{
			var input = Console.ReadLine().Split(' ');
			int a = int.Parse(input[0]);
			int b = int.Parse(input[1]);

			double k = (double)(a + b) / 2.0;
			if (k == (int)k)
			{
				Console.WriteLine(k);
			}
			else
			{
				Console.WriteLine("IMPOSSIBLE");
			}
		}
	}
}