#include<stdio.h>
int n,i,j,v[10000],ok,q;
int main ()
{
FILE*f=fopen("dame.in","r");
FILE*g=fopen("dame.out","w");

fscanf (f,"%d",&n);

fclose(f);
if (n==2) fprintf(g,"1\n1 1");
   else

     if (n==3) fprintf(g,"2\n1 1\n2 3");

	  else
{
q=n/2;
j=2;
for (i=1;i<=q;i++)
      {v[i]=j;
       j+=2;}

if (n%12==3||n%12==9)
   {for (i=1;i<q;i++)
	 v[i]=v[i+1];
    v[q]=2;}

j=1;
for (i=q+1;i<=n;i++)
     {v[i]=j;
      j+=2;}

if (n%12==8)
    for (i=q+1;i<n;i+=2)
	 {ok=v[i];
	  v[i]=v[i+1];
	  v[i+1]=ok;}

if (n%12==2)
    {ok=v[q+1];
     v[q+1]=v[q+2];
     v[q+2]=ok;
     for (i=q+3;i<n;i++)
	  v[i]=v[i+1];
     v[n]=5;}

if (n%12==3||n%12==9)
    {for (i=q+1;i<n-1;i++)
	 v[i]=v[i+2];
     v[n-1]=1;
     v[n]=3;}

fprintf (g,"%d\n",n);
for (i=1;i<=n;i++)
     fprintf (g,"%d %d\n",i,v[i]);
}
fclose(g);
return 0;
}