import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();
		int minDate = 0;
		if (n == 2 || m == 2 || (m == 3 && n % m == 2)) {
			minDate = -1;
		} else if (n <= m){
			 minDate = 1;
		} else if (n % m == 0){
			for (int i = 0 ; i < n / m ; i++) {
				minDate++;
			}
		} else if (n % m >= 1) {
			for (int i = 0 ; i < (n / m) + 1 ; i++) {
				minDate++;
			}
		}
		System.out.println(minDate);
	}
}