package main

import (
	"fmt"
)

func resolve(R, C int64) int64 {
	if R == C {
		if R == 1 {
			return 0
		}
		if R%2 == 0 {
			return R*C/4 - 1
		}
		return R * C / 4
	}
	if R%2 != 0 && C%2 != 0 {
		return (R*C+1)/2 - 1
	}
	return (R*C)/2 - 1
}

func main() {
	var R, C int64
	fmt.Scanf("%d %d\n", &R, &C)
	fmt.Println(resolve(R, C))
}