#include<stdio.h>
main()
{
int a[1001],i,j,k,r,s,x,l,n,m,sp,valid;
freopen("dame.in","r",stdin);
freopen("dame.out","w",stdout);
fscanf(stdin,"%d",&n);
r=n%12;
if((r==3)||(r==9))
{ j=1;
  for(i=4;i<=n;i+=2)
   { a[j]=i; j++; }
  a[j]=2;
  j++;
}
else
{ j=1;
  for(i=2;i<=n;i+=2)
   { a[j]=i; j++; }
}
valid=0;
if(r==8)
{
  i=3;
  do{
     a[j]=i;j++;
     a[j]=i-2;j++;
     i+=4;
     }while(i<=n);
  j--;
  valid=1;
}
if(r==2)
{
   a[j]=3;j++;
   a[j]=1;j++;
   for(i=7;i<=n;i+=2)
    { a[j]=i; j++; }
   a[j]=5;
   valid=1;
}
if((r==3)||(r==9))
{
   for(i=5;i<=n;i+=2)
    { a[j]=i; j++; }
   a[j]=1;j++;
   a[j]=3;
   valid=1;
}
if(valid==0)
{
   for(i=1;i<=n;i+=2)
    { a[j]=i;j++; }
   j--;
}
fprintf(stdout,"%d\n",j);
for(i=1;i<=j;i++)
 fprintf(stdout,"%d %d\n",i,a[i]);






fclose(stdin);
fclose(stdout);
return 0;
}