You are given an integer N. Build an undirected graph with N vertices with indices 1 to N that satisfies the following two conditions:

* The graph is simple and connected.
* There exists an integer S such that, for every vertex, the sum of the indices of the vertices adjacent to that vertex is S.



It can be proved that at least one such graph exists under the constraints of this problem.

Constraints

* All values in input are integers.
* 3 \leq N \leq 100

Input

Input is given from Standard Input in the following format:


N


Output

In the first line, print the number of edges, M, in the graph you made. In the i-th of the following M lines, print two integers a_i and b_i, representing the endpoints of the i-th edge.

The output will be judged correct if the graph satisfies the conditions.

Example

Input

3


Output

2
1 3
2 3
import java.io.*;
import java.util.*;
import java.util.function.Function;

public class Main {

    static int N;

    public static void main(String[] args) {
        FastScanner sc = new FastScanner(System.in);
        N = sc.nextInt();

        List<Edge> ans = solve();
        System.out.println( ans.size() );
        PrintWriter pw = new PrintWriter(System.out);
        for (Edge edge : ans) {
            pw.println(edge.a + " " + edge.b);
        }
        pw.flush();
    }

    static List<Edge> solve() {
        if( N % 2 == 0 ) {
            return even();
        } else {
            return odd();
        }
    }

    static List<Edge> odd() {
        List<Edge> edge = new ArrayList<>();
        for (int i = 1; i < N; i++) {
            for (int j = i+1; j < N; j++) {
                if( i + j != N ) {
                    edge.add( new Edge(i, j) );
                }
            }
            edge.add( new Edge(i, N) );
        }
        return edge;
    }

    static List<Edge> even() {
        List<Edge> edge = new ArrayList<>();
        for (int i = 1; i <= N; i++) {
            for (int j = i+1; j <= N; j++) {
                if( i + j != N+1 ) {
                    edge.add( new Edge(i, j) );
                }
            }
        }
        return edge;
    }

    static class Edge {
        int a, b;

        public Edge(int a, int b) {
            this.a = a;
            this.b = b;
        }
    }

    @SuppressWarnings("unused")
    static class FastScanner {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

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

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

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

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

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

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

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

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

    static <A> void writeLines(A[] as, Function<A, String> f) {
        PrintWriter pw = new PrintWriter(System.out);
        for (A a : as) {
            pw.println(f.apply(a));
        }
        pw.flush();
    }

    static void writeLines(int[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (int a : as) pw.println(a);
        pw.flush();
    }

    static void writeLines(long[] as) {
        PrintWriter pw = new PrintWriter(System.out);
        for (long a : as) pw.println(a);
        pw.flush();
    }

    static int max(int... as) {
        int max = Integer.MIN_VALUE;
        for (int a : as) max = Math.max(a, max);
        return max;
    }

    static int min(int... as) {
        int min = Integer.MAX_VALUE;
        for (int a : as) min = Math.min(a, min);
        return min;
    }

    static void debug(Object... args) {
        StringJoiner j = new StringJoiner(" ");
        for (Object arg : args) {
            if (arg instanceof int[]) j.add(Arrays.toString((int[]) arg));
            else if (arg instanceof long[]) j.add(Arrays.toString((long[]) arg));
            else if (arg instanceof double[]) j.add(Arrays.toString((double[]) arg));
            else if (arg instanceof Object[]) j.add(Arrays.toString((Object[]) arg));
            else j.add(arg.toString());
        }
        System.err.println(j.toString());
    }
}
