#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int n;
  cin >> n;
  vector<pair<int, int>> v;
  for (int i = 0; i < n; ++i) {
    for (int j = i + 1; j < n - i; ++j) {
      v.push_back({ i, j });
    }
  }
  cout << v.size() << '\n';
  for (auto [x, y] : v) cout << x + 1 << ' ' << y + 1 << '\n';
  return 0;
}