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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;

public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int ab=sc.nextInt();
    int bc=sc.nextInt();
    int ca=sc.nextInt();
    // ac*ac-(bc+x)*(bc+x)=h*h
    // ab*ab-x*x=h*h
    // ac*ac-ab*ab-bc*bc-2*bc*x=0
    // 2bc*x=ac*ac-ab*ab-bc*bc
    int x=(ca*ca-ab*ab-bc*bc)/(2*bc);
    double high=Math.sqrt(ab*ab-x*x);

    int ans=(int)(bc*high/2);

    System.out.println(ans);
  }
}