#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

int main(){

    int N, M;
    cin >> N;
    vector<pair<int, int>> v;

    for (int i=0; i<N/2; i++){
        M = N-i;
        for (int j=0; j<N-1-2*i; j++) v.push_back({M, M-1-j});
    }

    cout << v.size() << endl;
    for (auto [x, y] : v) cout << x << " " << y << endl;

    return 0;
}