#include<fstream>
using namespace std;
int n,i,j,a[1001],k,m;
int main()
{
	ifstream f("dame.in");
	ofstream g("dame.out");
	f>>n;
	if(n==2)
	{
		g<<1<<"\n";
		g<<1<<" "<<1;
	}
	else
		if(n==3)
		{
			g<<2<<"\n";
			g<<1<<" "<<1<<"\n";
			g<<2<<" "<<3;
		}
		else
		{
		//m=1;
		i=1;
		while(i<n)
		{
			m+=2;
			a[i]=m;
			i++;
		}
		i=(n/2)+1;
		m=1;
		while(i<=n)
		{
			a[i]=m;
			i++;
			m+=2;
		}
		g<<n<<"\n";
		for(i=1;i<=n;i++)
			g<<i<<" "<<a[i]<<"\n";
		}
		return 0;
}