package main

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

var scanner *bufio.Scanner

func main() {
	n, m := Int(), Int()
	if n%2 == 1 {
		fmt.Println(-1)
	} else if n <= m {
		fmt.Println(1)
	} else if n/2 > m {
		fmt.Println(-1)
	} else {
		fmt.Println(2)
	}
}

func init() {
	scanner = bufio.NewScanner(os.Stdin)
	scanner.Buffer([]byte{}, math.MaxInt64)
	scanner.Split(bufio.ScanWords)
}

func Int() int {
	scanner.Scan()
	ret, _ := strconv.ParseInt(scanner.Text(), 10, 0)
	return int(ret)
}

func Ints(n int) []int {
	ret := make([]int, n)
	for i := 0; i < n; i++ {
		ret[i] = Int()
	}
	return ret
}

type Number interface {
	~int | ~int8 | ~int16 | ~int32 | ~int64 | ~float32 | ~float64
}

func Max[T Number](a, b T) T {
	if a < b {
		return b
	}
	return a
}

func Min[T Number](a, b T) T {
	if a < b {
		return a
	}
	return b
}

func Abs[T Number](a T) T {
	if a < 0 {
		return -a
	}
	return a
}