import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import static java.lang.System.in;

public class No531 {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));

        String[] inputs = reader.readLine().split(" ");
        int n = Integer.parseInt(inputs[0]);
        int m = Integer.parseInt(inputs[1]);

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

                if ((n / 2) % m == 0) {
                    System.out.println((n / 2) / m + 1);
                } else {
                    System.out.println((n / 2) / m + 2);

                }

            } else {
                System.out.println(-1);
            }

        }

    }
}