#include <bits/stdc++.h>
using namespace std;
using namespace std;
const int MOD = 1000000007;
const int MAX = 1e6 + 5;
const int LOGMAX = 20;
int main() {
  int n;
  cin >> n;
  ;
  cout << ((n + 1) / 2) * (n / 2) << endl;
  for (int i = 1; i <= n; i += 2) {
    for (int j = 2; j <= n; j += 2) {
      cout << i << ' ' << j << endl;
      ;
    }
  }
}
