#include <vector>
#include <iostream>
#define MAXN 3000

using namespace std;

int N, mosse=0; int sol[2][MAXN];

int ordina(int n, int o);

int main() {
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    cin>>N;
    int n=N;
    ordina(n, 2*n+1);
    cout<<mosse<<N<<endl;
    for(int i=0; i<mosse; i++){
        cout<<sol[0][i]<<" "<<sol[1][i]<<endl;
    }

}

int ordina(int n, int o){
    if(n>3){
        sol[0][mosse]=n; sol[1][mosse]=o;
        sol[0][mosse+1]=o-2; sol[1][mosse+1]=n;
        mosse+=2;
        ordina(n-1, o-2);
        return 0;
    }else{
        sol[0][mosse]=2; sol[1][mosse]=7;
        mosse++;
        sol[0][mosse]=6; sol[1][mosse]=2;
        mosse++;
        sol[0][mosse]=4; sol[1][mosse]=6;
        mosse++;
        sol[0][mosse]=7; sol[1][mosse]=4;
        mosse++;
        return 0;
    }
}