#include <bits/stdc++.h> using namespace std; typedef long long int ll; typedef pair<ll,ll> P; typedef vector<ll> VI; typedef vector<VI> VVI; #define REP(i,n) for(ll i=0;i<(n);i++) #define ALL(v) v.begin(),v.end() constexpr ll MOD=1000000007; constexpr ll INF=2e18; int main(){ int n; cin >> n; vector<P> ans; VI c(n,0); REP(i,n){ for(int j=i+1;j<n;j++){ if(c[i]>=n-i-1) break; ans.push_back({i,j}); c[i]++, c[j]++; } } cout << ans.size() << endl; for(auto p:ans) cout << p.first+1 << " " << p.second+1 << endl; }