Problem:
Score : 100 points

Problem Statement

There is a right triangle ABC with ∠ABC=90°.

Given the lengths of the three sides, |AB|,|BC| and |CA|, find the area of the right triangle ABC.

It is guaranteed that the area of the triangle ABC is an integer.

Constraints

1 \leq |AB|,|BC|,|CA| \leq 100

All values in input are integers.

The area of the triangle ABC is an integer.

Input

Input is given from Standard Input in the following format:

|AB| |BC| |CA|

Output

Print the area of the triangle ABC.

Sample Input 1

3 4 5

Sample Output 1

6

This triangle has an area of 6.

Sample Input 2

5 12 13

Sample Output 2

30

This triangle has an area of 30.

Sample Input 3

45 28 53

Sample Output 3

630

This triangle has an area of 630.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
class Program
{
    static void Main()
    {
        //入力
        //int n = 100;
        /*
        long n = long.Parse(Console.ReadLine());
        long[] input = new long[5];
        for(int a = 0; a < 5; a++)
        {
            input[a]=long.Parse(Console.ReadLine());
        }
        */
        int[] input = Console.ReadLine().Split().Select(i => int.Parse(i)).ToArray();
        Console.WriteLine((int)input[0]*input[1]/2);


        /*
        //処理速度計測
        var sw = new System.Diagnostics.Stopwatch();
        sw.Start();
        //解法
 
        //処理速度表示
        sw.Stop();
        TimeSpan ts = sw.Elapsed;
        Console.WriteLine((int)(Math.Floor((ts).TotalMilliseconds))+"ms");
        */
        Console.ReadLine();

    }
}