#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;
using ll = long long;
using ull = unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)


int main(){
  int N; cin >> N;

  vector<pair<int,int>> edges;
  for(int i=1; i<=N; i++){
    for(int j=i+1; j<=N; j++){
      if(i+j >= N+1) edges.push_back({i,j});
    }
  }

  cout << edges.size() << "\n";
  for(auto e : edges) cout << e.first << " " << e.second << "\n";
  return 0;
}


struct ios_do_not_sync{
  ios_do_not_sync(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
  }
} ios_do_not_sync_instance;