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();
		sc.close();

		int n2 = n / 2;
		List<String> list = new ArrayList<>();
		int x = n - 1;
		for (int i = 1; i <= n2; i++) {
			for (int j = 1; j <= x; j++) {
				list.add(i + " " + (i + j));
			}
			x -= 2;
		}

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