#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int test = 1;
  while (test--) {
    int n;
    cin >> n;
    if (n == 1 || n == 2) {
      cout << 0 << endl;
      continue;
    } else if (n == 3) {
      cout << 1 << endl;
      cout << 1 << " " << 2 << endl;
      continue;
    } else if (n == 4) {
      cout << 2 << endl;
      cout << 1 << " " << 2 << endl;
      cout << 3 << " " << 4 << endl;
      continue;
    }
    long long int temp = pow(2, floor(log2(n)));
    vector<pair<long long int, long long int> > op1, op2;
    int adder = n - temp;
    for (int gap = 1; gap < temp; gap *= 2) {
      int cnt = (temp / 2) / gap;
      for (int j = 0; cnt > 0; j += 2) {
        cnt--;
        for (int k = (j * gap + 1); k <= ((j + 1) * gap) && (k + gap) <= temp;
             k++) {
          op1.push_back({k, k + gap});
        }
      }
    }
    for (int gap = 1; gap < temp; gap *= 2) {
      int cnt = (temp / 2) / gap;
      for (int j = 0; cnt > 0; j += 2) {
        cnt--;
        for (int k = (j * gap + 1 + adder);
             k <= ((j + 1) * gap + adder) && (k + gap) <= n; k++) {
          op2.push_back({k, k + gap});
        }
      }
    }
    cout << op1.size() + op2.size() << endl;
    for (int i = 0; i < op1.size(); i++) {
      cout << op1[i].first << " " << op1[i].second << endl;
    }
    for (int i = 0; i < op2.size(); i++) {
      cout << op2[i].first << " " << op2[i].second << endl;
    }
  }
  return 0;
}
