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#include <iostream>
#include <vector>
#include <utility>
#include <algorithm>
using namespace std;


int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
    int k;
    cin >> k;
    if(k == 3)
    { cout 
       << 6 << endl
       << 3 << " " << 5 << endl
       << 6 << " " << -1 << endl
       << 2 << " " << 6 << endl
       << 2 << " "  << 6 << endl
        << 6 << " " << -1 << endl
        << -1 << " " << -1 << endl;
        return 0;
    }

    cout << k + 1 << endl;
    int countr = 0;
    for(int i = 1; i < k + 1; ++i)
    {
        if(countr != k - 1)
        {
            cout << i + 1 << " " << k + 1 << endl;
            countr++;
        } 
        else cout << i + 1 << " " << -1 << endl;
    }
    cout << -1 << " " << -1 << endl;


    

    
}