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 <= m) {
			minDate = 1;
		} else if (n % m != 0 || n / m > m && n % m == 0) {
			minDate = -1;
		} else if (n / m <= m && n % 2 == 0) {
			minDate = n / m;
		}
		System.out.println(minDate);
	}
}