#include <iostream>
#include <vector>

using namespace std;

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

    vector<pair<int, int>> es;
    int l = 1, r = n;
    while (l < r) {
        for (int i = l; i < r; ++i) es.emplace_back(i, r);
        ++l, --r;
    }

    cout << es.size() << "\n";
    for (auto [u, v] : es) cout << u << " " << v << "\n";
}

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

    solve();

    return 0;
}