# include <fstream>
# include <cstdio>
# include <iostream>
# define DIM 1024
using namespace std;
int n, c[DIM], rez, x[DIM], y[DIM];

int main ()
{
	ifstream fin ("dame.in");
	fin>>n;
	int j=-1, s=1;
	for(int i=1;i<=n && s!=3;++i)
	{
		j+=2;
		if (j<=n)
			x[++rez]=i, y[rez]=j;
		else if (s==1)
			j=2, s=2, x[++rez]=i, y[rez]=j;
			else s=3;
	}
	freopen("dame.out", "w", stdout);
	printf("%d\n", rez);
	for(int i=1;i<=rez;++i)
		printf("%d %d\n", x[i], y[i]);
	return 0;
}