#include <cstdio>
using namespace std;
int main()
{
    int
    freopen("triang.in","r",stdin);
    freopen("triang.out","w",stdout);
    scanf("%d",&n);
    if(n<=3)
		printf("0");
	else
		if(n==4)
			printf("2\n1 3\n2 4");
		else
		{
			printf("%d\n",n*(n-2)*(n*4))/2);
			for(i1=1;i1<=n;i1++)
			{
				for(i2=i1+2;i2<=n-2;i2++)
				{
					printf("%d %d ",i1,i2);
				}
			}
		}
    return 0;
}