package yukicoder;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		@SuppressWarnings("resource")
		Scanner sc =  new Scanner(System.in);
		String moment = sc.nextLine();
		String[] spl = moment.split(" ");
		int animal = Integer.parseInt(spl[0]);
		int boat = Integer.parseInt(spl[1]);
		if(animal < 3 || boat < 3){
			System.out.println(-1);
			return;
		}
		int numDay = -1;
		for(int i = boat ; i > 2 ; i--){
			if((animal % i) != 2){
				numDay = animal / i;
				if((animal % i) != 0){
					numDay++;
				}
				break;
			}
		}
		System.out.println(numDay);
	}

}