#include<bits/stdc++.h>
#include "trilib.h"
using namespace std;
#define vec vector
#define int long long
int n;
bool on_the_right(int i, int j, int k) {
if(i==j || j == k || i == k) return false;
return is_clockwise(i+1, j+1, k+1);
}
bool is_on_convex_hull(int i) {
int mx = i == 0 ? 1 : 0;
for(int j = 0; j<n; j++) {
if(j == i || j == mx) continue;
if(on_the_right(i, mx, j)) mx = j;
}
for(int j = 0; j<n; j++) {
if(j == i || j == mx) continue;
if(on_the_right(i, mx, j)) return false;
}
return true;
}
vec<int> convex_hull_from_point(int p, vec<int> all_points) {
sort(all_points.begin(), all_points.end(), [&](int i, int j) { return on_the_right(p, j, i); });
vec<int> stk{p};
all_points.push_back(p);
for(int p_nxt : all_points) {
while(stk.size() > 1 && on_the_right(stk[stk.size()-2], stk[stk.size()-1], p_nxt)) {
stk.pop_back();
}
stk.push_back(p_nxt);
}
stk.pop_back();
return stk;
}
int32_t main() {
n = get_n();
int p1 = 0;
int p2 = 1;
vec<int> ps_r{};
vec<int> ps_l{};
for(int i = 2; i<n; i++) {
if(on_the_right(p1, p2, i)) ps_r.push_back(i);
else ps_l.push_back(i);
}
vec<int> ch_l = convex_hull_from_point(p1, ps_l);
vec<int> ch_r = convex_hull_from_point(p2, ps_r);
int l_i = 0;
int r_i = 0;
vec<int> chnxt(n, -1);
for(int i = 0; i<ch_l.size(); i++) {
chnxt[ch_l[i]] = ch_l[(i+1)%ch_l.size()];
}
for(int i = 0; i<ch_r.size(); i++) {
chnxt[ch_r[i]] = ch_r[(i+1)%ch_r.size()];
}
while(1) {
int l_i_nxt = (l_i+1)%ch_l.size();
int r_i_nxt = (r_i-1+ch_r.size())%ch_r.size();
if(on_the_right(ch_r[r_i], ch_l[l_i], ch_l[l_i_nxt])) {
l_i = l_i_nxt;
continue;
}
if(on_the_right(ch_r[r_i], ch_l[l_i], ch_r[r_i_nxt])) {
r_i = r_i_nxt;
continue;
}
chnxt[ch_r[r_i]] = ch_l[l_i];
break;
}
l_i = 0;
r_i = 0;
while(1) {
int l_i_nxt = (l_i-1+ch_l.size())%ch_l.size();
int r_i_nxt = (r_i+1)%ch_r.size();
if(on_the_right(ch_l[l_i], ch_r[r_i], ch_l[l_i_nxt])) {
l_i = l_i_nxt;
continue;
}
if(on_the_right(ch_l[l_i], ch_r[r_i], ch_r[r_i_nxt])) {
r_i = r_i_nxt;
continue;
}
chnxt[ch_l[l_i]] = ch_r[r_i];
break;
}
vec<int> ch{ch_l[0]};
for(int i = 0; i<n*2; i++) {
ch.push_back(chnxt[ch.back()]);
}
reverse(ch.begin(), ch.end());
vec<bool> used(n);
int ans = 0;
for(int p : ch) {
if(used[p]) break;
used[p] = true;
ans++;
}
give_answer(ans);
return 0;
}
