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 }