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