codeforces/java/50A.java

package com.company;

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
	// write your code here
        Scanner sc = new Scanner(System.in);
        int m = sc.nextInt();
        int n = sc.nextInt();
        int out = m*n/2;
        System.out.print(out);
    }
}