백준 #15700번 타일 채우기 4

import java.util.Scanner;
import java.math.BigInteger;
public class Main {
	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		BigInteger n = sc.nextBigInteger();
		BigInteger m = sc.nextBigInteger();
		
		System.out.println(n.multiply(m).divide(BigInteger.valueOf(2)));
		sc.close();
	}
}