#include <iostream>
#include <fstream>

using namespace std;
ifstream fin("dame.in");
ofstream fout("dame.out");
int main()
{
    double n;
    fin>>n;
    if(n==1 || n==2)
    {
        fout<<1<<endl;
        fout<<1<<" "<<1<<endl;
    }
    else if(n==3)
    {
        fout<<2<<endl;
        fout<<1<<" "<<2<<endl;
        fout<<3<<" "<<1<<endl;
    }
    else{
    if(int(n)%4==0)
        fout<<n<<endl;
    else
        fout<<n-1<<endl;
    for(int i=1;i<=n/2;i++)
            fout<<i<<" "<<2*i<<endl;
    if(int((n+4)/6)!=(n+4)/6)
    {
        int j=1, i=n/2+1;
        while(j<=n)
        {
            fout<<i<<" "<<j<<endl;
            j+=2;
            i++;
        }
    }
    else
    {
        int i=n/2+1,j=3;
        while(j<=n)
        {
            fout<<i<<" "<<j<<endl;
            fout<<i+1<<" "<<j-2<<endl;
            i+=2;
            j+=4;
        }
    }
    }
    return 0;
}