package main

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

var sc = bufio.NewScanner(os.Stdin)

func Scanner() string {
	sc.Scan()
	return sc.Text()
}

func main() {
	buf := make([]byte, 0)
	sc.Buffer(buf, 100000007)
	sc.Split(bufio.ScanWords)
	a, _ := strconv.Atoi(Scanner())
	b, _ := strconv.Atoi(Scanner())
	ans := -1
	if gcd(a, b) > 1 {
		fmt.Println(ans)
		return
	} else {
		ans = (a - 1) * (b - 1) / 2
	}
	fmt.Println(ans)
}

func gcd(a int, b int) int {
	if b == 0 {
		return a
	}
	return gcd(b, a%b)
}