#include<stdio.h>
int main(){
int n, i, j;
FILE*fi=freopen("dame.in","r",stdin);
FILE*fo=freopen("dame.out","w",stdout);
scanf("%d",&n);
if(n==1||n==2)
printf("1\n1 1\n");
else if(n==3)
printf("2\n1 1\n2 3\n");
else{
printf("%d\n",n);
if(n%2){
printf("%d %d\n",n,n);
--n;
}
if(n%6==2){
for(i=n/2,j=1;i<=n;i+=2,++j)
printf("%d %d\n%d %d\n",i,j,n+1-i,n+1-j);
for(i=2-i%2;j<=n/2;i+=2,++j)
printf("%d %d\n%d %d\n",i,j,n+1-i,n+1-j);
}
else
for(i=1;i<=n/2;++i)
printf("%d %d\n%d %d\n",i,2*i,n+1-i,n+1-2*i);
}
return 0;
}