#include <bits/stdc++.h>
using namespace std;
void debug_out() { cout << '\n'; }
template <typename T, typename... R>
void debug_out(const T &f, const R &...r) {
  cout << f << " ";
  debug_out(r...);
}
template <typename A, typename B>
ostream &operator<<(ostream &out, const pair<A, B> &a) {
  out << "(" << a.first << "," << a.second << ")";
  return out;
}
template <typename Tuple, size_t N>
struct tuple_print {
  static void print(const Tuple &t, std::ostream &os) {
    tuple_print<Tuple, N - 1>::print(t, os);
    os << ", " << std::get<N - 1>(t);
  }
};
template <typename Tuple>
struct tuple_print<Tuple, 1> {
  static void print(const Tuple &t, std::ostream &os) {
    os << "(" << std::get<0>(t);
  }
};
template <typename... Args>
ostream &operator<<(ostream &os, const tuple<Args...> &t) {
  tuple_print<decltype(t), sizeof...(Args)>::print(t, os);
  return os << ")";
}
template <typename T>
ostream &operator<<(ostream &out, const vector<T> &a) {
  out << "[";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "]";
  return out;
}
template <typename T, class Cmp>
ostream &operator<<(ostream &out, const set<T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &v : a) {
    out << (first ? "" : ", ");
    out << v;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename U, typename T, class Cmp>
ostream &operator<<(ostream &out, const map<U, T, Cmp> &a) {
  out << "{";
  bool first = true;
  for (auto &p : a) {
    out << (first ? "" : ", ");
    out << p.first << ":" << p.second;
    first = 0;
  }
  out << "}";
  return out;
}
template <typename Arg1>
void __f(const char *name, Arg1 &&arg1) {
  cerr << name << ": " << arg1 << endl;
}
template <typename Arg1, typename... Args>
void __f(const char *names, Arg1 &&arg1, Args &&...args) {
  const char *comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << ": " << arg1 << " |";
  __f(comma + 1, args...);
}
const long long mod = 1e9 + 7;
const int N = 1e6 + 100;
int T;
int n, tot, x[N], y[N];
void solve(int l, int r) {
  if (l == r - 1) {
    tot++, x[tot] = l, y[tot] = r;
    return;
  }
  int mid = l + r >> 1;
  solve(l, mid), solve(mid + 1, r);
  for (int i = l, j = mid + 1; i <= mid && j <= r; ++i, ++j) {
    tot++, x[tot] = i, y[tot] = j;
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin >> n;
  if (n == 1 || n == 2) {
    cout << 0 << '\n';
    return 0;
  }
  int base = 1;
  while (base <= n) base *= 2;
  base /= 2;
  solve(1, base);
  solve(n - base + 1, n);
  cout << tot << '\n';
  for (int i = 1; i <= tot; i++) cout << x[i] << ' ' << y[i] << '\n';
  return 0;
}
