#include<fstream.h>

long n,i,x,aux,a[1001];

int main()

{
ifstream f("dame.in");
ofstream g("dame.out");
f>>n;
if (n==2 || n==3) g<<n-1;
else g<<n;
g<<'\n';
x=2;
for (i=1;i<=n/2;i++)
    {
    a[i]=x;
    x+=2;
    }
if (n%12==3 || n%12==9)
   {
        for (i=2;i<=n/2;i++)
            a[i-1]=a[i];
        a[n/2]=2;
   }
x=1;
for (i=n/2+1;i<=n;i++)
    {
    a[i]=x;
    x+=2;
    }
if (n%12==8)
   {
        for (i=n/2+1;i<n;i++)
            {
                aux=a[i];
                a[i]=a[i+1];
                a[i+1]=aux;
                i++;
            }
   }
if (n%12==2)
   {
   a[n/2+1]=3;
   a[n/2+2]=1;
   for (i=n/2+3;i<n;i++)
       a[i]=a[i+1];
   a[n]=5;
   }
if (n%12==3 || n%12==9)
   {
        for (i=n/2+3;i<=n;i++)
            a[i-2]=a[i];
   a[n-1]=1;
   a[n]=3;
   }
for (i=n;i>=1;i--)
    g<<n-i+1<<' '<<a[i]<<'\n';
g.close();
return 0;
}