#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("ciur.in");
ofstream fout("ciur.out");
int n, sir[2000004], suma = 0;



int main()
{
    fin >> n;
    for(int i = 1; i <= n; ++i) {
        sir[i] = i;
    }
    for(int i = 2; i <= n; ++i) {
        if(sir[i] != -1) {
                cout << sir[i] << " ";
                suma++;
            for(int j = i * 2 - 1; j <= n; ++j) {
                    cout << sir[i] << " " << j << endl;
                if(j % sir[i] == 0) {
                    sir[j] = -1;
                }
            }
        }
    }
    fout << suma;
    return 0;
}