#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
vector<pair<int, int>> ans;
int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n / 2; i++) {
		for (int j = i + 1; j < n - i; j++) {
			ans.emplace_back(j, n - i);
		}
	}
	cout << ans.size() << endl;
	for (int i = 0; i < ans.size(); i++) {
		cout << ans[i].first << " " << ans[i].second << endl;
	}
}