#include <bits/stdc++.h>
using namespace std;
int main() {
    int n;
    cin >> n;
    vector<pair<int, int>> edges;
    for (int i = 0; i * 2 < n; i++) {
        for (int j = i + 1; j < n - i; j++) {
            edges.push_back({i + 1, j + 1});
        }
    }
    int m = edges.size();
    cout << m << endl;
    for (auto edge : edges) {
        int u = edge.first, v = edge.second;
        cout << u << " " << v << endl;
    }
}