package main

import (
	"fmt"
)

func main() {
	var R, C int64
	fmt.Scan(&R, &C)
	if R == C{
		if R%2==0 {
			fmt.Println(R/2*C/2-1)
		}else {
			fmt.Println(R/2*(C/2+1))
		}
	} else {
		fmt.Println((R*C+1)/2-1)
	}
}