import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int k = sc.nextInt();
		sc.close();

		if (k == 0) {
			System.out.println(-1);
			return;
		}

		List<String> list = new ArrayList<>();
		for (int i = 1; i <= k; i++) {
			list.add(i + " " + i);
		}
		for (int i = k + 1; i <= n; i++) {
			list.add("1 " + i);
			list.add(i + " " + i);
		}

		PrintWriter pw = new PrintWriter(System.out);
		pw.println(list.size());
		for (String s : list) {
			pw.println(s);
		}
		pw.flush();
	}
}