#include<fstream.h>

int main()
{
int n,i;
ifstream f ("dame.in");
ofstream g ("dame.out");
f>>n; f.close();
if (n==2) g<<1<<"\n"<<1<<' '<<1;
	else if (n==3) g<<2<<"\n"<<1<<' '<<1<<"\n"<<3<<' '<<2;
		else {
g<<n<<"\n";
g<<1<<' '<<n/2+n%2<<"\n";
for (i=2;i<=n;i++)
	if (i%2==0)  {
		     if (n/2%3==1) if (i<=4) g<<i<<' '<<n/2+n%2+1+(2-i/2)<<"\n";
					else g<<i<<' '<<n-i/2+3<<"\n";
		     if (n/2%3!=1) g<<i<<' '<<n-i/2+1<<"\n";
		     }
		else g<<i<<' '<<n/2+n%2-i/2<<"\n";  }
g.close();
return 0;
}