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.Text;
    using System.Collections.Generic;
    using static System.Math;
    using System.Linq;
    using static System.Array;
    
    namespace ConsoleApp2
    {
    class Program
    {
        static string S;
        static long ans = 0;
        static long N;
        static long M;
        static Node[] Nodes;
        static void Main(string[] args)
        {

            //var N = long.Parse(Console.ReadLine());
            var ABC = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
            var A = ABC[0];
            var B = ABC[1];
            var C = ABC[2];

            if (A > B)
            {
                Swap(ref A, ref B);
            }

            if (B > C)
            {
                Swap(ref C, ref B);
            }
            Console.WriteLine(A*B/2);
        }
        static void Swap<T>(ref T a, ref T b)
        {
            var tmp = a;
            a = b;
            b = tmp;
        }

    }

    class Node
    {
        long num = -1;
        public List<long> adj;
        public Node(long inp)
        {
            this.num = inp;
            adj = new List<long>();
        }
    }
}