#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

#define mkp make_pair
#define mkt make_tuple
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
template <class T> void chmin(T &a, const T &b) {
    if (a > b) a = b;
}
template <class T> void chmax(T &a, const T &b) {
    if (a < b) a = b;
}

void solve() {
    int N;
    cin >> N;

    vector<pair<int, int>> output;
    for (int i = 0; i < N; i++) {
        int rig = N - i;
        for (int j = i + 1; j < rig; j++) {
            output.emplace_back(i, j);
        }
    }

    cout << output.size() << "\n";
    for (auto [a, b] : output) cout << a + 1 << " " << b + 1 << "\n";
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    solve();

    return 0;
}