#include <bits/stdc++.h> #include "trilib.h" #define int long long #define sz(a) (int)a.size() #define all(a) a.begin(), a.end() #define rall(a) a.rbegin(), a.rend() using namespace std; signed main() { ios_base::sync_with_stdio(false); cin.tie(nullptr); int n=get_n(); int root=1; int x1,x2; vector<int> a; vector<int> b; a.push_back(2); b.push_back(2); for (int i = 3; i <= n; i++) { if(i==root) continue; if(is_clockwise(1,2,i)) a.push_back(i); else b.push_back(i); } sort(all(a), [root](int _a, int _b){ return is_clockwise(root,_a,_b); }); sort(all(b), [root](int _a, int _b){ return !is_clockwise(root,_a,_b); }); vector<int> convA; convA.push_back(root); convA.push_back(a[0]); for (int i = 1; i < sz(a); i++) { while(true){ x2=convA.back(); convA.pop_back(); x1=convA.back(); if(is_clockwise(x1,x2,a[i])){ convA.push_back(x2); break; } } convA.push_back(a[i]); } reverse(all(convA)); convA.pop_back(); reverse(all(convA)); convA.push_back(root); vector<int> convB; convB.push_back(root); convB.push_back(b[0]); for (int i = 1; i < sz(b); i++) { while(true){ x2=convB.back(); convB.pop_back(); x1=convB.back(); if(!is_clockwise(x1,x2,b[i])){ convB.push_back(x2); break; } } convB.push_back(b[i]); } reverse(all(convB)); convB.pop_back(); reverse(all(convB)); convB.push_back(root); reverse(all(convB)); vector<int> convAB,conv; for (int i = 1; i < sz(convA); i++) { if(sz(convAB)>0&&convAB.back()==convA[i]) continue; convAB.push_back(convA[i]); } for (int i = 1; i < sz(convB); i++) { if(sz(convAB)>0&&convAB.back()==convB[i]) continue; convAB.push_back(convB[i]); } int sAB=sz(convAB); for (int i = 0; i < sAB; i++) { convAB.push_back(convAB[i]); } conv.push_back(convAB[0]); conv.push_back(convAB[1]); for (int i = 2; i < sz(convAB); i++) { while(true){ x2=conv.back(); conv.pop_back(); x1=conv.back(); if(is_clockwise(x1,x2,convAB[i])){ conv.push_back(x2); break; } } conv.push_back(convAB[i]); } vector<int> cnt(n+1); int sm=0; for (int i = 0; i < sz(conv); i++) { cnt[conv[i]]++; if(cnt[conv[i]]==2) sm++; } give_answer(sm); return 0; }