#include <cstdio>

#define Nmax 4096

#define sch(x, y) x ^= y ^= x ^= y

int v[Nmax], n, cap, coada;

int main()
{
	freopen("dame.in", "r", stdin);
	freopen("dame.out", "w", stdout);
	
	int i;
	
	scanf("%d", &n);
	
	if (n == 1 || n == 2)
	{
		printf("1\n");
		printf("1 1\n");
		return 0;
	}
	if (n == 3)
	{
		printf("2\n");
		printf("1 1\n3 2\n");
		return 0;
	}
	
	printf("%d\n", n);
	cap = 1;
	for (i = 2; i <= n; i += 2)
		v[++coada] = i;
	if (n % 12 == 3 || n %12 == 9)
	{
		for (i = 1; i < coada; ++i)
			v[i] = v[i+1];
		v[coada] = 2;
	}
	for (i = cap; i <= coada; ++i)
		printf("%d %d\n", i, v[i]);
		
	cap = 1; coada = 0;
	for (i = 1; i <= n; i += 2)
		v[++coada] = i;
	if (n % 12 == 8)
		for (i = 2; i <= coada; i+=2)
			sch(v[i-1], v[i]);
	if (n % 12 == 2)
	{
		sch(v[1], v[2]);
		for (i = 3; i < coada; ++i)
			v[i] = v[i+1];
		v[coada] = 5;
	}
	if (n % 12 == 3 || n%12 ==9)
	{
		for (i = 1; i <= coada-2; ++i)
			v[i] = v[i+2];
		v[coada-1] = 1;
		v[coada] = 3;
	}
	for (i = cap; i <= coada; ++i)
		printf("%d %d\n", n/2+i, v[i]);
	
	return 0;
}