#include<fstream>
using namespace std;
int n, i, nr;
ifstream fin("dame.in");
ofstream fout("dame.out");
int main(){
	fin>> n;
	if(n >= 4){
		fout<< n <<"\n";
		nr = 2;
		for(i = 1; i <= n / 2; i++){
			fout<< i <<" "<< nr <<"\n";
			nr += 2;
		}
		nr = 1;
		for(i = n / 2 + 1; i <= n; i++){
			fout<< i <<" "<< nr <<"\n";
			nr += 2;
		}
	}
	else{
		if(n == 1 || n == 2){
			fout<< 1 <<"\n"<< 1 <<" "<< 1 <<"\n"; 
		}
		else{
			fout<< 2 <<"\n"<< 1 <<" "<< 1 <<"\n"<< 2 <<" "<< 3 <<"\n";
		}
	}
	return 0;
}