#include "trilib.h"
#ifndef EVAL
#include "trilib.c"
#endif
#include "bits/stdc++.h"
using namespace std;
#define ar array
typedef int64_t ll;
//~ void WA(){
//~ give_answer(n + 1);
//~ exit(0);
//~ }
vector<int> Sort(int l, int r){
if(l == r) return {l};
int m = (l + r) >> 1;
auto lx = Sort(l, m), rx = Sort(m + 1, r);
vector<int> res;
l = 0, r = 0;
while(l < (int)lx.size() || r < (int)rx.size()){
if(l < (int)lx.size() && r < (int)rx.size()){
if(is_clockwise(1, lx[l], rx[r])){
res.push_back(lx[l++]);
} else {
res.push_back(rx[r++]);
}
} else if(l < (int)lx.size()){
res.push_back(lx[l++]);
} else {
res.push_back(rx[r++]);
}
}
return res;
}
signed main(){
//~ ios::sync_with_stdio(0); cin.tie(0);
int n = get_n();
vector<int> p = Sort(2, n);
int m = p.size(), sz = 0;
vector<int> last, is(n, 1);
is[0] = 0;
for(int i=0;i<m;i++){
while(sz > 1 && !is_clockwise(last[sz - 2], last[sz - 1], p[i])){
is[last.back() - 1] = 0;
last.pop_back();
sz--;
}
last.push_back(p[i]);
sz++;
}
auto check = [&](int a, int b){
ar<int, 2> cc {};
for(int i=1;i<=n;i++){
if(i == a || i == b) continue;
cc[is_clockwise(a, b, i)]++;
}
return (cc[0] == 0 || cc[1] == 0);
};
if(check(1, last[0]) && check(1, last[sz - 1])){
give_answer((int)last.size() + 1);
return 0;
}
for(int i=0;i<m;i++){
while(sz > 1 && !is_clockwise(last[sz - 2], last[sz - 1], p[i])){
is[last.back() - 1] = 0;
last.pop_back();
sz--;
}
last.push_back(p[i]);
sz++;
}
int sum = accumulate(is.begin(), is.end(), 0);
give_answer(sum);
return 0;
}
