#include "trilib.h"
#include <bits/stdc++.h>
using namespace std;
typedef long long un;
typedef vector<un> vuc;
typedef set<un> sun;
#define vec vector
#define REP(i, a, b) for (un i = (un)a; i < (un)b; i++)
#define FEAC(i, a) for (auto&& i : a)
#define ALL(x) (x).begin(), (x).end()
un N;
vuc horni;
vuc dolni;
vuc cely;
vuc obal;
int main(){
N = get_n();
REP(i, 3, N+1){
if (not is_clockwise(1, 2, i)) horni.push_back(i);
else dolni.push_back(i);
}
horni.push_back(2);
dolni.push_back(2);
sort(ALL(horni), [](un x, un y){return is_clockwise(1, x, y); });
sort(ALL(dolni), [](un x, un y){return is_clockwise(1, x, y); });
FEAC(a, horni) cely.push_back(a);
cely.pop_back();
FEAC(a, dolni) cely.push_back(a);
obal.push_back(1);
FEAC(a, cely){
while((obal.size() != 1) and (not is_clockwise(obal[obal.size()-2], obal[obal.size()-1], a))){
obal.pop_back();
}
obal.push_back(a);
}
un ign = 0;
// while (not is_clockwise(obal[obal.size()-1], obal[ign], obal[ign+1]))
// {
// ign++;
// }
give_answer(obal.size()-ign);
}
