#include <fstream>

using namespace std;

ifstream fin ("mesaj4.in");
ofstream fout ("mesaj4.out");

int N, M;

int main() {
	fin >> N;

	fout << 2*(N-1) << "\n";
	for(int i = 1; i <= 2*(N-1); ++i) {
		fout << 1 << " " << 2 << "\n";
	}

}