#include <bits/stdc++.h>
#pragma GCC diagnostic ignored "-Wsign-compare"
#pragma GCC diagnostic ignored "-Wsign-conversion"
#define NDEBUG
#define SHOW(...) static_cast<void>(0)
//!===========================================================!//
//!  dP     dP                          dP                    !//
//!  88     88                          88                    !//
//!  88aaaaa88a .d8888b. .d8888b. .d888b88 .d8888b. 88d888b.  !//
//!  88     88  88ooood8 88'  '88 88'  '88 88ooood8 88'  '88  !//
//!  88     88  88.  ... 88.  .88 88.  .88 88.  ... 88        !//
//!  dP     dP  '88888P' '88888P8 '88888P8 '88888P' dP        !//
//!===========================================================!//
template <typename T, typename A>
std::istream& operator>>(std::istream& is, std::vector<T, A>& v)
{
    for (auto& e : v) { is >> e; }
    return is;
}
template <typename T>
T read()
{
    T v;
    return std::cin >> v, v;
}
template <typename T>
std::vector<T> readVec(const std::size_t l)
{
    std::vector<T> v(l);
    for (auto& e : v) { std::cin >> e; }
    return v;
}
using ld = long double;
using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
constexpr unsigned int MOD = 1000000007;
template <typename T>
constexpr T INF = std::numeric_limits<T>::max() / 4;
template <typename F>
constexpr F PI = static_cast<F>(3.1415926535897932385);
std::mt19937 mt{std::random_device{}()};
template <typename T>
bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); }
template <typename T>
bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); }
template <typename T>
std::vector<T> Vec(const std::size_t n, T v) { return std::vector<T>(n, v); }
template <class... Args>
auto Vec(const std::size_t n, Args... args) { return std::vector<decltype(Vec(args...))>(n, Vec(args...)); }
template <typename T>
constexpr T popCount(const T u)
{
#ifdef __has_builtin
    return u == 0 ? T(0) : (T)__builtin_popcountll(u);
#else
    unsigned long long v = static_cast<unsigned long long>(u);
    return v = (v & 0x5555555555555555ULL) + (v >> 1 & 0x5555555555555555ULL), v = (v & 0x3333333333333333ULL) + (v >> 2 & 0x3333333333333333ULL), v = (v + (v >> 4)) & 0x0F0F0F0F0F0F0F0FULL, static_cast<T>(v * 0x0101010101010101ULL >> 56 & 0x7f);
#endif
}
template <typename T>
constexpr T log2p1(const T u)
{
#ifdef __has_builtin
    return u == 0 ? T(0) : T(64 - __builtin_clzll(u));
#else
    unsigned long long v = static_cast<unsigned long long>(u);
    return v = static_cast<unsigned long long>(v), v |= (v >> 1), v |= (v >> 2), v |= (v >> 4), v |= (v >> 8), v |= (v >> 16), v |= (v >> 32), popCount(v);
#endif
}
template <typename T>
constexpr T clog(const T v) { return v == 0 ? T(0) : log2p1(v - 1); }
template <typename T>
constexpr T msbp1(const T v) { return log2p1(v); }
template <typename T>
constexpr T lsbp1(const T v)
{
#ifdef __has_builtin
    return __builtin_ffsll(v);
#else
    return v == 0 ? T(0) : popCount((v & (-v)) - T(1)) + T(1);
#endif
}
template <typename T>
constexpr bool ispow2(const T v) { return popCount(v) == 1; }
template <typename T>
constexpr T ceil2(const T v) { return v == 0 ? T(1) : T(1) << log2p1(v - 1); }
template <typename T>
constexpr T floor2(const T v) { return v == 0 ? T(0) : T(1) << (log2p1(v) - 1); }
//!=================================================!//
//!   .88888.                             dP        !//
//!  d8'   '88                            88        !//
//!  88        88d888b. .d8888b. 88d888b. 88d888b.  !//
//!  88   YP88 88'  '88 88'  '88 88'  '88 88'  '88  !//
//!  Y8.   .88 88       88.  .88 88.  .88 88    88  !//
//!   '88888'  dP       '88888P8 88Y888P' dP    dP  !//
//!                              88                 !//
//!                              dP                 !//
//!=================================================!//
struct Graph
{
    Graph(const std::size_t v) : V{v}, edge(v), rev_edge(v) {}
    void addEdge(const std::size_t from, const std::size_t to, const bool bi = false)
    {
        assert(from < V), assert(to < V);
        edge[from].push_back(to), rev_edge[to].push_back(from);
        if (bi) { addEdge(to, from, false); }
    }
    const std::vector<std::size_t>& operator[](const std::size_t i) const { return assert(i < V), edge[i]; }
    friend std::ostream& operator<<(std::ostream& os, const Graph& g)
    {
        os << "[\n";
        for (std::size_t i = 0; i < g.V; i++) {
            for (const std::size_t to : g.edge[i]) { os << i << "->" << to << "\n"; }
        }
        return (os << "]\n");
    }
    std::size_t V;
    std::vector<std::vector<std::size_t>> edge, rev_edge;
};
using Tree = Graph;
//!============================================!//
//!    8888ba.88ba             oo              !//
//!    88  '8b  '8b                            !//
//!    88   88   88  .d8888b.  dP  88d888b.    !//
//!    88   88   88  88'  '88  88  88'  '88    !//
//!    88   88   88  88.  .88  88  88    88    !//
//!    dP   dP   dP  '88888P8  dP  dP    dP    !//
//!============================================!//
int main()
{
    const int N = read<int>();
    auto G = Graph(3);
    G.addEdge(2, 0, true), G.addEdge(2, 1, true);
    for (int i = 3; i < N; i++) {
        Graph NG = Graph(i + 1);
        if (i % 2 == 1) {
            for (int j = 1; j < i; j++) { NG.addEdge(0, j, true); }
            for (int j = 0; j < i; j++) {
                for (const int to : G[j]) { NG.addEdge(j + 1, to + 1, false); }
            }
        } else {
            for (int j = 0; j < i; j++) {
                NG.addEdge(i, j, true);
                for (const int to : G[j]) { NG.addEdge(j, to, false); }
            }
        }
        G = NG;
    }
    std::vector<int> u, v;
    for (int i = 0; i < N; i++) {
        for (const int to : G[i]) {
            if (to <= i) { continue; }
            u.push_back(i + 1), v.push_back(to + 1);
        }
    }
    std::cout << u.size() << std::endl;
    for (int i = 0; i < u.size(); i++) { std::cout << u[i] << " " << v[i] << std::endl; }
    return 0;
}


Measured memory_bytes: 768