#include <bits/stdc++.h>
using namespace std;

void fast_io() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
}

int main() {
	fast_io();
	int n;
	cin >> n;
	vector<pair<int, int>> ans;
	for (int i = 1; i <= n; i++) {
		for (int j = i + 1; j <= n - i + 1; j++) {
			ans.push_back({i, j});
		}
	}
	cout << ans.size() << '\n';
	for (auto [a, b] : ans) {
		cout << a << ' ' << b << '\n';
	}
}