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: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
class Program
 {
        static void Main(string[] args)
        {
        　　int A,B,K=-1;
            string[] str = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得
            A = int.Parse(str[0]);  //数値で受け取りたい場合は変換する
            B = int.Parse(str[1]);
            for(int i = 0; i < Math.Pow(10, 9); i++)
            {
                if(Math.Abs(A - i) == Math.Abs(B - i))
                {
                    K = i;
                    break;
                }
            }
            if(K == -1)
            {
                Console.Write("IMPOSSIBLE");
            }
            else
            {
                Console.Write(K);
            }
        }
 }