#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <queue>
#include <iomanip>
#include <set>
#include <tuple>
#define mkp make_pair
#define mkt make_tuple
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
const ll MOD=1e9+7;
template<class T> void chmin(T &a,const T &b){if(a>b) a=b;}
template<class T> void chmax(T &a,const T &b){if(a<b) a=b;}

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  cin>>N;
  
  //vector<vector<int>> exi_edge(N,vector<int> (N,0));
  vector<int> deg(N,0);
  vector<pair<int,int>> output;

  for(int d=1;d<N;d++){
    vector<int> cnt(N,0);
    rep(i,d) cnt[deg[i]]++;

    vector<int> used(N,0);
    rep(i,d){
      if(deg[i]==d-1){
        output.push_back(mkp(i,d));
        cnt[deg[i]]--;

        deg[i]++;
        deg[d]++;
        used[i]=1;

        cnt[deg[i]]++;
        break;
      }
    }

    for(int k=1;k+1<d;k++){
      if(cnt[k]<=1) continue;
      rep(i,d){
        if(deg[i]!=k) continue;
        if(used[i]) continue;

        output.push_back(mkp(i,d));
        cnt[deg[i]]--;

        deg[i]++;
        deg[d]++;
        used[i]=1;

        cnt[deg[i]]++;
        break;
      }
    }
  }

  cout<<output.size()<<"\n";
  for(auto [a,b]:output) cout<<a+1<<" "<<b+1<<"\n";

  return 0;
}