#include<iostream> #include<vector> using namespace std; int N; main() { cin>>N; vector<int>cnt(N,0); vector<pair<int,int> >E; for(int i=0;i<N;i++) { int t=N-i-1; if(cnt[i]>t)break; t-=cnt[i]; for(int j=0;j<t;j++) { E.push_back(make_pair(i,i+j+1)); cnt[i]++; cnt[i+j+1]++; } } cout<<E.size()<<endl; for(pair<int,int>e:E)cout<<e.first+1<<" "<<e.second+1<<endl; }