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

int main() {
    int n;
    cin >> n;
    int times = 1;
    int m = 0;
    vector<pair<int, int>> out;
    for(int i=1;i<=n;i++) {
        if(i + 1 > n - i + 1) break;
        for(int j=i+1;j<=n-i+1;j++) {
            out.emplace_back(make_pair(i, j));
            m++;
        }
    }

    cout << m << endl;

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

    return 0;
}