#include <stdio.h>

long n,i,p,q;

int main(){
    freopen("dame.in","r",stdin);
    freopen("dame.out","w",stdout);
    
    scanf("%ld %ld",&n);
    if (n==1||n==2)printf("%ld\n%ld%ld\n",1,1,1);
    if (n==3)printf("%ld\n%ld %ld\n%ld %ld\n",2,1,1,2,3);
    if (n>=4){
       printf("%ld\n",n);
       p=(n+1)/2;
       if (n%2==1)for (i=1;i<=p;i++)printf("%ld %ld\n",i,2*i-1);
       else for (i=1;i<=p;i++)printf("%ld %ld\n",i,2*i);
       q=2;
       for (i=p+1;i<=n;i++,q+=2)printf("%ld %ld\n",i,q);
    }
    
return 0;
}