// IO library
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>
#include <random>
#include <limits>

// contancer library
#include <bitset>
#include <deque>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>
#include <functional>

using ll = long long;
using ld = long double;

/* ----- class ----- */

template <class Cost = int, class Cap = int>
struct Edge {
    int from, to, rev;
    Cost cost;
    Cap cap;
    Edge(int from = -1, int to = -1, Cost cost = 1, Cap cap = 0, int rev = -1)
        : from(from), to(to), cost(cost), cap(cap), rev(rev){};

    bool operator<(const Edge<Cost>& e) const { return this->cost < e.cost; }
    bool operator>(const Edge<Cost>& e) const { return this->cost > e.cost; }
};

template <class Cost = int, class Cap = int>
class Graph {
public:
    int size;
    std::vector<std::vector<Edge<Cost, Cap>>> path;

    explicit Graph(int N = 0) : size(N), path(size) {}
    void span(int u, int v, Cost cost = 1, Cap cap = 0) {
        path[u].push_back(Edge<Cost, Cap>(u, v, cost, cap));
    }
    std::vector<Edge<Cost, Cap>> operator[](int v) const { return path[v]; }
};

/* ----- Output Functions for Debugging ----- */

template <class T>
std::ostream& operator<<(std::ostream& os, std::vector<T> v);
template <class T>
std::ostream& operator<<(std::ostream& os, std::set<T> v);
template <class L, class R>
std::ostream& operator<<(std::ostream& os, std::pair<L, R> p);
template <class K, class T>
std::ostream& operator<<(std::ostream& os, std::map<K, T> v);
template <class T>
std::ostream& operator<<(std::ostream& os, std::queue<T> q);
template <class T>
std::ostream& operator<<(std::ostream& os, std::priority_queue<T> q);
template <class T>
std::ostream& operator<<(std::ostream& os, std::priority_queue<T, std::vector<T>, std::greater<T>> q);
template <class T, class U>
std::ostream& operator<<(std::ostream& os, Edge<T, U> e);

template <class T>
std::ostream& operator<<(std::ostream& os, std::vector<T> v) {
    os << "[";
    for (auto vv : v) os << vv << ",";
    return os << "]";
}
template <class T>
std::ostream& operator<<(std::ostream& os, std::set<T> v) {
    os << "{";
    for (auto vv : v) os << vv << ",";
    return os << "}";
}
template <class L, class R>
std::ostream& operator<<(std::ostream& os, std::pair<L, R> p) {
    return os << "(" << p.first << "," << p.second << ")";
}
template <class K, class T>
std::ostream& operator<<(std::ostream& os, std::map<K, T> v) {
    os << "{";
    for (auto vv : v) os << vv << ",";
    return os << "}";
}
template <class T>
std::ostream& operator<<(std::ostream& os, std::queue<T> q) {
    os << "[";
    while (!q.empty()) {
        os << q.front() << ",";
        q.pop();
    }
    return os << "]";
}
template <class T>
std::ostream& operator<<(std::ostream& os, std::priority_queue<T> q) {
    os << "{";
    while (!q.empty()) {
        os << q.top() << ",";
        q.pop();
    }
    return os << "}";
}
template <class T>
std::ostream& operator<<(std::ostream& os, std::priority_queue<T, std::vector<T>, std::greater<T>> q) {
    os << "{";
    while (!q.empty()) {
        os << q.top() << ",";
        q.pop();
    }
    return os << "}";
}
template <class T, class U>
std::ostream& operator<<(std::ostream& os, Edge<T, U> e) {
    return os << "(" << e.from << "->" << e.to << ":" << e.cost << "," << e.cap << ")";
}

/* ----- Short Functions ----- */

template <class T>
T Vec(T v) { return v; }

template <class T, class... Ts>
auto Vec(size_t l, Ts... ts) {
    return std::vector<decltype(Vec<T>(ts...))>(l, Vec<T>(ts...));
}

template <class T>
inline T sq(T a) { return a * a; }

template <class T>
inline T iceil(T n, T d) { return (n + d - 1) / d; }

template <class T>
T gcd(T a, T b) {
    while (b > 0) {
        a %= b;
        swap(a, b);
    }
    return a;
}

template <class T, class U>
T ipow(T b, U n) {
    T ret = 1;
    while (n > 0) {
        if (n & 1) ret *= b;
        n >>= 1;
        b *= b;
    }
    return ret;
}

// 0-indexed
template <class T, class U>
inline T kthbit(T a, U k) { return (a >> k) & 1; }

template <class T, class U>
inline T mask(T a, U k) { return a & ((1 << k) - 1); }

/* ----- Constants ----- */

// const int INF = std::numeric_limits<int>::max();
// const ll INF = std::numeric_limits<ll>::max();
// const ld PI = acos(-1);
// const ld EPS = 1e-10;
// std::mt19937 mt(ll(time(0)));


int main() {
    int N;
    std::cin >> N;
    int S = (N % 2 == 0 ? N + 1 : N);

    std::vector<int> u, v;
    for (int i = 1; i <= N; ++i) {
        for (int j = i + 1; j <= N; ++j) {
            if (i + j != S) {
                u.push_back(i);
                v.push_back(j);
            }
        }
    }

    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: 384