#include <bits/stdc++.h>
// created [2019/11/10] 21:03:43
#pragma GCC diagnostic ignored "-Wsign-compare"
#pragma GCC diagnostic ignored "-Wsign-conversion"

using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
using uint = unsigned int;
using usize = std::size_t;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
template<typename T> constexpr T popcount(const T u) { return u ? static_cast<T>(__builtin_popcountll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T log2p1(const T u) { return u ? static_cast<T>(64 - __builtin_clzll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T msbp1(const T u) { return log2p1(u); }
template<typename T> constexpr T lsbp1(const T u) { return __builtin_ffsll(u); }
template<typename T> constexpr T clog(const T u) { return u ? log2p1(u - 1) : static_cast<T>(u); }
template<typename T> constexpr bool ispow2(const T u) { return u and (static_cast<u64>(u) & static_cast<u64>(u - 1)) == 0; }
template<typename T> constexpr T ceil2(const T u) { return static_cast<T>(1) << clog(u); }
template<typename T> constexpr T floor2(const T u) { return u == 0 ? static_cast<T>(0) : static_cast<T>(1) << (log2p1(u) - 1); }
template<typename T> constexpr bool btest(const T mask, const usize ind) { return static_cast<bool>((static_cast<u64>(mask) >> ind) & static_cast<u64>(1)); }
template<typename T> void bset(T& mask, const usize ind) { mask |= (static_cast<T>(1) << ind); }
template<typename T> void breset(T& mask, const usize ind) { mask &= ~(static_cast<T>(1) << ind); }
template<typename T> void bflip(T& mask, const usize ind) { mask ^= (static_cast<T>(1) << ind); }
template<typename T> void bset(T& mask, const usize ind, const bool b) { (b ? bset(mask, ind) : breset(mask, ind)); }
template<typename T> constexpr T bcut(const T mask, const usize ind) { return ind == 0 ? static_cast<T>(0) : static_cast<T>((static_cast<u64>(mask) << (64 - ind)) >> (64 - ind)); }
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); }
constexpr unsigned int mod                  = 1000000007;
template<typename T> constexpr T inf_v      = std::numeric_limits<T>::max() / 4;
template<typename Real> constexpr Real pi_v = Real{3.141592653589793238462643383279502884};

template<typename T>
T read()
{
    T v;
    return std::cin >> v, v;
}
template<typename T, typename... Args>
auto read(const usize size, Args... args)
{
    std::vector<decltype(read<T>(args...))> ans(size);
    for (usize i = 0; i < size; i++) { ans[i] = read<T>(args...); }
    return ans;
}
template<typename... Types>
auto reads() { return std::tuple<std::decay_t<Types>...>{read<Types>()...}; }
#    define SHOW(...) static_cast<void>(0)
template<typename T>
T make_v(const T v) { return v; }
template<typename... Args>
auto make_v(const std::size_t size, Args... args) { return std::vector<decltype(make_v(args...))>(size, make_v(args...)); }

template<typename Cost>
class base_graph
{
public:
    using cost_type = Cost;
    base_graph(const usize sz) : sz{sz}, edges(sz), rev_edges(sz) {}
    void add_edge(const usize from, const usize to, const Cost cost, const bool bi = false)
    {
        assert(from < sz), assert(to < sz);
        edges[from].push_back(edge{from, to, cost}), rev_edges[to].push_back(edge(to, from, cost));
        if (bi) { add_edge(to, from, cost, false); }
    }
    struct edge
    {
        edge(const usize from, const usize to, const Cost cost) : from{from}, to{to}, cost{cost} {}
        usize from, to;
        Cost cost;
        bool operator<(const edge& e) const { return cost != e.cost ? cost < e.cost : to < e.to; }
    };
    std::vector<edge>& operator[](const usize i) { return assert(i < sz), edges[i]; }
    const std::vector<edge>& operator[](const usize i) const { return assert(i < sz), edges[i]; }
    const std::vector<std::vector<edge>>& rev_edge() const { return rev_edges; }
    std::vector<std::vector<edge>>& rev_edge() { return rev_edges; }
    friend std::ostream& operator<<(std::ostream& os, const base_graph& g)
    {
        os << "[\n";
        for (usize i = 0; i < g.sz; i++) {
            for (const auto& e : g.edges[i]) { os << i << "->" << e.to << ":" << e.cost << "\n"; }
        }
        return (os << "]\n");
    }
    static usize to(const edge& e) { return e.to; }
    usize size() const { return sz; }

private:
    const usize sz;
    std::vector<std::vector<edge>> edges, rev_edges;
};
template<>
class base_graph<void>
{
public:
    base_graph(const usize sz) : sz{sz}, edges(sz), rev_edges(sz) {}
    void add_edge(const usize from, const usize to, const bool bi = false)
    {
        assert(from < sz), assert(to < sz);
        edges[from].push_back(to), rev_edges[to].push_back(from);
        if (bi) { add_edge(to, from, false); }
    }
    std::vector<usize>& operator[](const usize i) { return assert(i < sz), edges[i]; }
    const std::vector<usize>& operator[](const usize i) const { return assert(i < sz), edges[i]; }
    const std::vector<std::vector<usize>>& rev_edge() const { return rev_edges; }
    std::vector<std::vector<usize>>& rev_edge() { return rev_edges; }
    friend std::ostream& operator<<(std::ostream& os, const base_graph& g)
    {
        os << "[\n";
        for (usize i = 0; i < g.sz; i++) {
            for (const usize to : g.edges[i]) { os << i << "->" << to << "\n"; }
        }
        return (os << "]\n");
    }
    static usize to(const usize e) { return e; }
    usize size() const { return sz; }

private:
    const usize sz;
    std::vector<std::vector<usize>> edges, rev_edges;
};
using graph = base_graph<void>;
using tree  = graph;
template<typename Cost>
using cost_graph = base_graph<Cost>;
template<typename Cost>
using cost_tree = cost_graph<Cost>;
int main()
{
    auto f = [&](auto&& self, const usize n) -> graph {
        graph g(n);
        if (n == 3) {
            return g.add_edge(2, 0), g.add_edge(2, 1), g;
        }
        if (n % 2 == 0) {
            const auto gsub = self(self, n - 1);
            for (usize i = 0; i < n - 1; i++) {
                for (const usize to : gsub[i]) { g.add_edge(i + 1, to + 1); }
            }
            for (usize i = 1; i < n - 1; i++) { g.add_edge(i, 0); }
        } else {
            const auto gsub = self(self, n - 1);
            for (usize i = 0; i < n - 1; i++) {
                for (const usize to : gsub[i]) { g.add_edge(i, to); }
            }
            for (usize i = 0; i < n - 1; i++) { g.add_edge(i, n - 1); }
        }
        return g;
    };
    const auto n = read<usize>();
    const auto g = f(f, n);
    using pii    = std::pair<usize, usize>;
    std::vector<pii> es;
    for (usize i = 0; i < n; i++) {
        for (const usize to : g[i]) { es.push_back({i + 1, to + 1}); }
    }
    std::cout << es.size() << std::endl;
    for (const auto& e : es) { std::cout << e.first << " " << e.second << std::endl; }
    return 0;
}


Measured memory_bytes: 768