#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; }