#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits>
#include <map>
#include <queue>
#include <random>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <sys/timeb.h>
#include <vector>

using namespace std;

#define repr(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) repr(i, 0, n)
#define reprrev(i, a, b) for (int i = (int)(b)-1; i >= (int)(a); i--)
#define reprev(i, n) reprrev(i, 0, n)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define mp make_pair
#define mt make_tuple
#define INF 1050000000
#define INFR INT_MAX
#define INFL (long long)(4e18)
#define INFLR LLONG_MAX
#define EPS (1e-10)
#define MOD 1000000007
//#define MOD 998244353
#define PI 3.141592653589793238
#define RMAX 4294967295

typedef long long ll;
typedef pair< int, int > Pi;
typedef pair< ll, ll > Pll;
typedef pair< double, double > Pd;
typedef vector< int > vi;
typedef vector< ll > vll;
typedef vector< bool > vb;
typedef vector< char > vc;
typedef vector< string > vs;
typedef vector< double > vd;
typedef vector< Pi > vPi;
typedef vector< Pll > vPll;
typedef vector< Pd > vPd;
typedef vector< vector< int > > vvi;
typedef vector< vector< bool > > vvb;
typedef vector< vector< ll > > vvll;
typedef vector< vector< char > > vvc;
typedef vector< vector< string > > vvs;
typedef vector< vector< double > > vvd;
typedef vector< vector< Pi > > vvPi;
typedef vector< vector< vector< int > > > vvvi;
typedef vector< vector< vector< ll > > > vvvll;
typedef vector< vector< vector< Pi > > > vvvPi;
typedef vector< vector< vector< vector< Pi > > > > vvvvPi;
typedef priority_queue< int, vector< int >, greater< int > > pqli;
typedef priority_queue< ll, vector< ll >, greater< ll > > pqlll;
typedef priority_queue< Pi, vector< Pi >, greater< Pi > > pqlP;
template < class T > using vec = vector< T >;
template < class T > using pql = priority_queue< T, vector< T >, greater< T > >;

// vvvvvvvvvvvvvvvvvvvvvvv debug output vvvvvvvvvvvvvvvvvvvvvvv
// vector
template < typename T > istream &operator>>(istream &is, vector< T > &vec) {
    for (T &x : vec)
        is >> x;
    return is;
}
// pair
template < typename T, typename U >
ostream &operator<<(ostream &os, pair< T, U > &pair_var) {
    os << "(" << pair_var.first << "," << pair_var.second << ")";
    return os;
}
// pair
template < typename T, typename U >
ostream &operator<<(ostream &os, const pair< T, U > &pair_var) {
    os << "(" << pair_var.first << "," << pair_var.second << ")";
    return os;
}
// vector
template < typename T >
ostream &operator<<(ostream &os, const vector< T > &vec) {
    os << "{";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
}
// map
template < typename T, typename U >
ostream &operator<<(ostream &os, map< T, U > &map_var) {
    os << "{";
    repi(itr, map_var) {
        os << "(" << itr->first << "," << itr->second << ")";
        itr++;
        if (itr == map_var.end())
            os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
// set
template < typename T > ostream &operator<<(ostream &os, set< T > &set_var) {
    os << "{";
    repi(itr, set_var) {
        os << "  " << *itr << endl;
        itr++;
        if (itr == set_var.end())
            os << ", ";
        itr--;
    }
    os << "}";
    return os;
}

#define DUMPOUT cerr

void dump_func() {
    DUMPOUT << endl;
}
template < class Head, class... Tail >
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) {
        DUMPOUT << ", ";
    }
    dump_func(std::move(tail)...);
}

#ifdef DEBUG_
#define DEB
#else
#define DEB if (false)
#endif

#ifdef DEBUG_
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ";                                                         \
    dump_func(__VA_ARGS__)
#else
#define dump(...)
#endif

// ^^^^^^^^^^^^^^^^^^^^^^^ debug output ^^^^^^^^^^^^^^^^^^^^^^^

string YN(bool y, int id = 0) {
    if (id)
        cout << id;
    return (y ? "YES" : "NO");
}
string yn(bool y, int id = 0) {
    if (id)
        cout << id;
    return (y ? "Yes" : "No");
}
string ON(bool y, int id = 0) {
    if (id)
        cout << id;
    return (y ? "OK" : "NG");
}

int dir4[4][2] = {{0, -1}, {-1, 0}, {1, 0}, {0, 1}};
int dir8[8][2] = {{-1, -1}, {0, -1}, {1, -1}, {-1, 0},
                  {1, 0},   {-1, 1}, {0, 1},  {1, 1}};
char dirchar[4] = {'<', '^', '>', 'v'};

// [a,b)
int irand(int a, int b) {
    static mt19937 Rand(static_cast< unsigned int >(time(nullptr)));
    uniform_int_distribution< int > dist(a, b - 1);
    return dist(Rand);
}

// [a,b)
double drand(int a, int b) {
    static mt19937 Rand(static_cast< unsigned int >(time(nullptr)));
    uniform_real_distribution< double > dist(a, b);
    return dist(Rand);
}

template < typename T > T max(vector< T > a) {
    return *max_element(all(a));
}
template < typename T > T min(vector< T > a) {
    return *min_element(all(a));
}
template < typename T > T sum(vector< T > a) {
    return accumulate(all(a), 0);
}

template < typename T > void add(map< T, int > &m, T item) {
    if (m.find(item) == m.end()) {
        m[item] = 1;
    } else {
        m[item]++;
    }
}

template < typename T, typename U > U get(map< T, U > m, T key, U def) {
    if (m.find(key) == m.end()) {
        return def;
    } else {
        return m[key];
    }
}

template < typename T > bool contains(set< T > t, const T &key) {
    return t.find(key) != t.end();
}

struct Edge {
    int from, to, cost;
    bool operator<(Edge e) {
        return cost < e.cost;
    }
};
using Graph = vec< vec< Edge > >;

ostream &operator<<(ostream &os, Edge &edge) {
    os << "(" << edge.from << "->" << edge.to << ":" << edge.cost << ")";
    return os;
}

//======================================================

int main(void) {
    int N;
    cin >> N;
    int sum = N - 1 - (N % 2);
    dump(sum);
    vPi edge;
    edge.reserve(N * (N + 1) / 2);
    rep(i, N) {
        repr(j, i + 1, N) {
            if (i + j != sum) {
                edge.push_back(mp(i, j));
            }
        }
    }
    cout << edge.size() << endl;
    for (auto e : edge) {
        cout << e.first + 1 << " " << e.second + 1 << endl;
    }

    return 0;
}

Measured memory_bytes: 256