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

public class Main {
    static FastScanner sc = new FastScanner(System.in);
    static PrintWriter pw = new PrintWriter(System.out);
    static StringBuilder sb = new StringBuilder();
    static long mod = (long) 1e9 + 7;

    public static void main(String[] args) throws Exception {
        solve();
        pw.flush();
    }
    
    public static void solve() {
        int N = sc.nextInt();
        int cnt = 0;
        for(int i = 1; i <= N; i++){
            for(int j = i+1; j <= N-i+1; j++){
                cnt++;
                sb.append(i + " " + j + "\n");
            }
        }
        pw.println(cnt);
        pw.println(sb.toString());
    }
    
    private static long rep2(long b, long n, long mod){
        if(n == 0) return 1;
        long bn = rep2(b,n/2,mod);
        if(n % 2 == 0){
            return (bn*bn)%mod;
        }else{
            return (bn*bn)%mod*b%mod;
        }
    }

    static class GeekInteger {
        public static void save_sort(int[] array) {
            shuffle(array);
            Arrays.sort(array);
        }

        public static int[] shuffle(int[] array) {
            int n = array.length;
            Random random = new Random();
            for (int i = 0, j; i < n; i++) {
                j = i + random.nextInt(n - i);
                int randomElement = array[j];
                array[j] = array[i];
                array[i] = randomElement;
            }
            return array;
        }

        public static void save_sort(long[] array) {
            shuffle(array);
            Arrays.sort(array);
        }

        public static long[] shuffle(long[] array) {
            int n = array.length;
            Random random = new Random();
            for (int i = 0, j; i < n; i++) {
                j = i + random.nextInt(n - i);
                long randomElement = array[j];
                array[j] = array[i];
                array[i] = randomElement;
            }
            return array;
        }

    }
}

class FastScanner {
    private BufferedReader reader = null;
    private StringTokenizer tokenizer = null;

    public FastScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
        tokenizer = null;
    }

    public FastScanner(FileReader in) {
        reader = new BufferedReader(in);
        tokenizer = null;
    }

    public String next() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public String nextLine() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken("\n");
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public double nextDouble() {
        return Double.parseDouble(next());
    }

    public String[] nextArray(int n) {
        String[] a = new String[n];
        for (int i = 0; i < n; i++)
            a[i] = next();
        return a;
    }

    public int[] nextIntArray(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = nextInt();
        return a;
    }

    public long[] nextLongArray(int n) {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = nextLong();
        return a;
    }
}