package main

import (
	"fmt"
	"math"
)

func main() {
	var r, c int
	_, _ = fmt.Scan(&r, &c)

	ans := r / 2 * c
	if r%2 == 1 {
		ans += int(math.Ceil(float64(c) / 2))
	}
	fmt.Println(ans - 1)
}