text: #define DEBUG 1
  
  /**
   * File    : B.cpp
   * Author  : Kazune Takahashi
   * Created : 3/23/2019, 10:22:47 PM
   * Powered by Visual Studio Code
   */
  
  #include <iostream>
  #include <iomanip>   // << fixed << setprecision(xxx)
  #include <algorithm> // do { } while ( next_permutation(A, A+xxx) ) ;
  #include <vector>
  #include <string> // to_string(nnn) // substr(m, n) // stoi(nnn)
  #include <complex>
  #include <tuple>
  #include <queue>
  #include <stack>
  #include <map> // if (M.find(key) != M.end()) { }
  #include <set>
  #include <functional>
  #include <random> // auto rd = bind(uniform_int_distribution<int>(0, 9), mt19937(10101010));
  #include <chrono> // std::chrono::system_clock::time_point start_time, end_time;
  // start = std::chrono::system_clock::now();
  // double elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end_time-start_time).count();
  #include <cctype>
  #include <cassert>
  #include <cmath>
  #include <cstdio>
  #include <cstdlib>
  using namespace std;
  
  typedef long long ll;
  
  // const int dx[4] = {1, 0, -1, 0};
  // const int dy[4] = {0, 1, 0, -1};
  
  // const int C = 1e6+10;
  // const ll M = 1000000007;
  
  int N;
  vector<vector<int>> V;
  typedef tuple<int, int> edge;
  vector<edge> E;
  
  void flush()
  {
    int S = V.size();
    for (auto i = 0; i < S; i++)
    {
      for (auto j = i + 1; j < S; j++)
      {
        for (auto x : V[i])
        {
          for (auto y : V[j])
          {
            E.push_back(edge(x, y));
          }
        }
      }
    }
    cout << E.size() << endl;
    for (auto x : E)
    {
      cout << get<0>(x) << " " << get<1>(x) << endl;
    }
  }
  
  int main()
  {
    cin >> N;
    if (N % 2 == 0)
    {
      for (auto i = 1; i <= N / 2; i++)
      {
        V.push_back({i, N + 1 - i});
      }
    }
    else
    {
      V.push_back({N});
      N--;
      for (auto i = 1; i <= N / 2; i++)
      {
        V.push_back({i, N + 1 - i});
      }
    }
    flush();
  }