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.Generic;
using System.Text.RegularExpressions;

namespace AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var ir = new Resolver(new ConsoleReader());
            Console.WriteLine(ir.Resolve());
        }
    }

    public class Resolver
    {
        IInputReader InputReader { get; }
        public List<string> Messages { get; set; } = new List<string>();

        int[] ls;

        public void ParseLines(){
            ls = InputReader.ReadLine().Split(' ').ToList().Select(x => int.Parse(x)).ToArray();
        }


        public string Resolve()
        {
            var result = Math.Sqrt((ls[0] + ls[1] + ls[2]) * (-ls[0] + ls[1] + ls[2]) * (ls[0] - ls[1] + ls[2]) * (ls[0] + ls[1] - ls[2])) / 4;

            return result.ToString();
        }



        public Resolver(IInputReader inputReader)
        {
            InputReader = inputReader;
            ParseLines();
        }

    }

    public interface IInputReader
    {
        string ReadLine();
    }

    class ConsoleReader : IInputReader
    {
        string IInputReader.ReadLine()
        {
            return Console.ReadLine();
        }
    }
}