#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    int times = 1;
    int m = 0;
    vector<pair<int, int>> out;
    if(n > 2) {
        m++;
        out.emplace_back(make_pair(2, 3));
    }
    int tmp = 3;

    for(int i=3;i<1000;i++) {
        if(tmp >= n) break;
        for(int j=0;j<times;j++) {
            if(tmp == n) break;
            m++;
            tmp++;
            out.emplace_back(make_pair(i, tmp));
        }
        times++;
    }

    cout << m << endl;
    for(int i=0;i<m;i++) {
        cout << out[i].first << ' ' << out[i].second << endl;
    }

    return 0;
}