import java.io.*;
import java.util.*;
import java.math.*;

class Main {
	
	public static void out (Object o) {
		System.out.println(o);
	}
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		String[] line = br.readLine().split(" ");
		BigInteger r = new BigInteger(line[0]);
		BigInteger c = new BigInteger(line[1]);
		BigInteger rc = r.multiply(c);
		BigInteger one = BigInteger.ONE;
		BigInteger two = BigInteger.valueOf(2);
		BigInteger four = BigInteger.valueOf(4);
		
		int f0 = r.compareTo(c);
		int f1 = rc.remainder(BigInteger.valueOf(2)).compareTo(BigInteger.ZERO);
		
		if (f0 == 0)
			out(f1 == 0 ? rc.divide(four).subtract(one) : rc.subtract(one).divide(four));
		else
			out(f1 == 0 ? rc.divide(two).subtract(one) : rc.subtract(one).divide(two));
	}
}