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

int main() {
	int N;
	cin >> N;
	auto dfs = [&](auto& self, int l, int r) -> void {
		if (r - l == 1) {
			return;
		}
		int m = (l + r) / 2;
		self(self, l, m);
		self(self, m, r);
		cout << l + 1 << ' ' << m + 1 << endl;
	};
	cout << 31 << endl;
	dfs(dfs, 0, 32);
	return 0;
}