#include <iostream>
#include <fstream>

using namespace std;
ifstream fin("dame.in");
ofstream fout("dame.out");
int main()
{
    int n,j=1;
    fin>>n;
    fout<<n<<endl;
    for(int i=1;i<=n/2;i++)
        fout<<i<<" "<<i*2<<endl;
    for(int i=n/2+1;i<=n;i++){
        fout<<i<<" "<<j<<endl;
        j+=2;
    }
    return 0;
}