package main

import "fmt"

func main() {
	var n, m int
	fmt.Scan(&n, &m)
	if m < 3 {
		fmt.Println("-1")
	} else {
		k := (n + (m - 1)) / m
		if k > 1 {
			k += ((k + (m - 1)) / m)
			k *= 2
		}
		fmt.Println(k)
	}
}