package main

import (
	"fmt"
)

func main() {
	var r, c, z int64
	fmt.Scan(&r, &c)
	n := r * c
	switch n % 4 {
	case 0:
		z = n / 4
	case 2:
		z = n / 2
	default:
		z = n/2 + n%2
	}
	fmt.Println(z - 1)
}