#include <bits/stdc++.h>
using namespace std;
int main(){
  int N;
  cin >> N;
  vector<int> u, v;
  int M = 0;
  for (int i = N - 1; i >= (N + 1) / 2; i--){
    for (int j = N - i; j < i; j++){
      u.push_back(i);
      v.push_back(j);
      M++;
    }
    u.push_back(i);
    v.push_back(N);
    M++;
  }
  cout << M << endl;
  for (int i = 0; i < M; i++){
    cout << u[i] << ' ' << v[i] << endl;
  }
}