#include<stdio.h>
int n,nr;
int v[1024];

void read()
{
	freopen("dame.in","r",stdin);
	freopen("dame.out","w",stdout);
	scanf("%d",&n);
	if(n==2)
	{
		printf("1\n1 1\n");
		return;
	}
	if(n==3)
	{
		printf("2\n1 1\n3 2\n");
		return;
	}
	printf("%d\n",n);
	int i,r=n%12;
	if(r==3 || r==9)
	{
		for(i=4;i<=n;i+=2)
			v[++nr]=i;
		v[++nr]=2;
	}
	else
		for(i=2;i<=n;i+=2)
			v[++nr]=i;
	if(r==8)
	{
		for(i=3;i<=n;i+=4)
		{
			v[++nr]=i;
			v[++nr]=i-2;
		}
	}
	else
		if(r==2)
		{
			v[++nr]=3;
			v[++nr]=1;
			for(i=7;i<=n;i+=2)
				v[++nr]=i;
			v[++nr]=5;
		}
		else
			if(r==3 || r==9)
			{
				for(i=5;i<=n;i+=2)
					v[++nr]=i;
				v[++nr]=1;
				v[++nr]=3;
			}
			else
				for(i=1;i<=n;i+=2)
					v[++nr]=i;
	for(i=1;i<=n;i++)
		printf("%d %d\n",i,v[i]);
}

int main()
{
	read();
	return 0;
}