package main

import (
	"bufio"
	"os"
	"strconv"
	"fmt"
)

// 参考 http://yukicoder.me/submissions/9329
var s = bufio.NewScanner(os.Stdin)

func next() string {
	s.Split(bufio.ScanWords)
	s.Scan()
	return s.Text()
}

func nextInt() int {
	i, e := strconv.Atoi(next())
	if e != nil {
		panic(e)
	}
	return i
}
func nextLong() int64 {
	i, e := strconv.ParseInt(next(), 10, 64)
	if e != nil {
		panic(e)
	}
	return i
}

func main() {
	R := nextLong()
	C := nextLong()
	ans := int64(0)
	n := R * C

	if R == C {
		ans = n/4+n%4
	}else {
		ans = n/2+n%2
	}
	fmt.Println(ans - 1)
}