#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
#define pb push_back
bool ccw(int x, int y, int z){
return is_clockwise(x, y, z);
}
int n, a, b;
bool cmp(int x, int y){
if (x == y) return 0;
return !ccw(a, x, y);
}
int main(){
n = get_n();
a = n - 1, b = n;
vector<int> v[2];
for (int i = 1; i <= n - 2; ++i)
v[!ccw(a, b, i)].pb(i);
for (int i = 0; i < 2; ++i){
sort(v[i].begin(), v[i].end(), cmp);
v[i].pb(b), swap(a, b);
}
vector<int> c[2];
for (int i = 0; i < 2; ++i){
c[i].pb(a);
for (int x: v[i]){
while ((int)(c[i].size()) >= 2 && ccw(c[i][(int)(c[i].size()) - 2], c[i].back(), x))
c[i].pop_back();
c[i].pb(x);
}
swap(a, b);
}
if ((int)(c[0].size()) == 2 || (int)(c[1].size()) == 2){
give_answer((int)(c[0].size()) + (int)(c[1].size()) - 2); return 0;
}
int l1 = 0, r1 = (int)(c[1].size()) - 2;
int l2 = (int)(c[0].size()) - 2, r2 = 0;
while (1){
if (ccw(c[1][r1], c[0][l1], c[0][l1 + 1])) ++l1;
else if (ccw(c[1][r1 - 1], c[1][r1], c[0][l1])) --r1;
else break;
}
while (1){
if (ccw(c[0][l2], c[1][r2], c[1][r2 + 1])) ++r2;
else if (ccw(c[0][l2 - 1], c[0][l2], c[1][r2])) --l2;
else break;
}
set<int> s;
for (int i = l1; i <= l2; ++i)
s.insert(c[0][i]);
for (int i = r2; i <= r1; ++i)
s.insert(c[1][i]);
give_answer((int)(s.size()));
return 0;
}
