Problem:
Score : 700 points

Problem Statement

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.

Sample Input 1

3

Sample Output 1

2
1 3
2 3

For every vertex, the sum of the indices of the vertices adjacent to that vertex is 3.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;

namespace AtCoder
{
    class B
    {
        public int index;
        public int num;
    }

    class Program
    {
        static void Main()
        {
            //ここから
            int N = IO.ReadLineToInt();
            int M = N % 2 == 0 ? (N - 2) * N / 2 : (N - 1) * (N - 1) / 2;
            int S = N % 2 == 0 ? (N + 1) * (N / 2 - 1) : N * (N / 2);

            List<Tuple<int, int>> ans = new List<Tuple<int, int>>();

            if (N % 2 == 0)
            {
                for(int i = 1; i <= N; i++)
                {
                    for(int j = i + 1; j <= N; j++)
                    {
                        if (i + j != N + 1)
                        {
                            ans.Add(new Tuple<int, int>(i,j));
                        }
                    }
                }
            }
            else
            {
                for (int i = 1; i < N; i++)
                {
                    for (int j = i + 1; j <= N; j++)
                    {
                        if (i + j != N )
                        {
                            ans.Add(new Tuple<int, int>(i, j));
                        }
                    }
                }
            }

            Console.WriteLine(M);
            for(int i = 0; i < M; i++)
            {
                Console.WriteLine(ans[i].Item1 + " " + ans[i].Item2);
            }

            //ここまで
            Console.ReadKey();
        }
    }
    
    class Func
    {
        long mod = 1000000007;
        static public void Swap<T>(ref T a, ref T b) { T temp = a; a = b; b = temp; }
        static public long GCD(int a, int b) { return b != 0 ? GCD(b, a % b) : a; }
        static public long LCM(int a, int b) { return (long)a * b / GCD(a, b); }
        static public int Factorial(int a) { return a == 1 ? a : a * Factorial(a - 1); }
        static public int Conbin(int n, int m) { return Factorial(n) / (Factorial(n - m) * Factorial(m)); }
        static public void Factoring(long num, ref long[] vs)
        {
            if (num == 1)
            {
                vs[1]++;
            }
            for (long i = 2; i < (long)Math.Sqrt(num); i++)
            {
                if (num % i == 0)
                {
                    vs[i]++;
                    Factoring(num / i, ref vs);
                }
            }
            vs[1]++;
        }
    }

    class IO
    {
        static public string ReadLineToString(){return Console.ReadLine();}
        static public int ReadLineToInt(){return int.Parse(Console.ReadLine());}
        static public long ReadLineToLong() { return long.Parse(Console.ReadLine()); }
        static public string[] ReadLineToStringArray(){return Console.ReadLine().Split(' ').ToArray();}
        static public int[] ReadLineToIntArray(){return Console.ReadLine().Split(' ').Select(int.Parse).ToArray();}
        static public long[] ReadLineToLongArray() { return Console.ReadLine().Split(' ').Select(long.Parse).ToArray(); }
        static public List<string> ReadLineToStringList(){return Console.ReadLine().Split(' ').ToList();}
        static public List<int> ReadLineToIntList(){return Console.ReadLine().Split(' ').Select(int.Parse).ToList();}
        static public List<long> ReadLineToLongList() { return Console.ReadLine().Split(' ').Select(long.Parse).ToList(); }   
    }
}