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;//外
using System.Linq;//Linq
public class ABC{

    public static void Main(){
        var line = Console.ReadLine();
        var l = line.Split(' ').Select(s => int.Parse(s)).ToArray();//Select,λ型
        int a = l[0];
        int b = l[1];
        if ( b > a ){
          var t = a;
          a =  b;
          b =  t;
        }
        if ((a-b)%2==1){//%の定義がa-(a/b)*b
            Console.WriteLine("IMPOSSIBLE");
        }
        else{
            Console.WriteLine((a+b)/2);
        }
    }
}