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; } }