#include<stdio.h>

int fct(long a,long b)
{if(a%2==0 && b%2==0)return 0;
if(a%3==0 && b%3==0)return 0;
if(a%5==0 && b%5==0)return 0;
if(a%7==0 && b%7==0)return 0;
if(a%11==0 && b%11==0)return 0;
if(a%13==0 && b%13==0)return 0;
if(a==b)return 0;
return 1;
}


int main()
{freopen("fractii.in","r",stdin);
freopen("fractii.out","w",stdout);
long i,j,n;
int k;
scanf("%ld",&n);
k=(2*int(n))-1;
for(i=2;i<=n;++i)
{for(j=2;j<=n;++j)
if (fct(i,j))
{++k;printf("%ld %ld \n",i,j);
}}
printf("%i",k);
return 0;
}