import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        StringBuilder sb = new StringBuilder();
        int count = 0;
        for (int i = 1; i < n - i + 1; i++) {
            for (int j = i + 1; j <= n - i + 1; j++) {
                sb.append(i).append(" ").append(j).append("\n");
                count++;
            }
        }
        System.out.println(count);
        System.out.print(sb);
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}