#include <iostream>
#include <vector>
using namespace std;
int main(){
	int n;cin>>n;
	int nw = n;
  vector<pair<int,int>> A;
	for(int i = 1; n >= i; i++){
		for(int j = i+1; nw >= j; j++){
      A.push_back({i,j});
		}
		nw--;
	}
  cout << A.size() << endl;
  for(int i = 0; A.size() > i; i++)cout << A[i].first << " " << A[i].second << endl;
}