#include<bits/stdc++.h>
#include "trilib.h"
#include<unordered_map>
#define rep(i,a,b) for(int i=int(a);i<int(b);i++)
#define rrep(i,a,b) for(int i=int(a);i>int(b);i--)
#define all(v) v.begin(),v.end()
#define trav(a,v) for(auto&a:v)
#define sz(a) a.size()
typedef long double ld;
using namespace std;
static const long long inf = 1e15;
typedef long long ll;
typedef unsigned long long ull;
int main() {
ll n;
n = get_n();
ll cur = 1;
ll nxt = 2;
vector<bool> v(n);
rep(i, 3, n + 1) {
if (is_clockwise(cur, nxt, i)) {
nxt = i;
}
}
v[nxt] = true;
cur = nxt;
rep(i, 3, n + 1) {
while (nxt == cur || v[nxt]) {
nxt++;
nxt %= n + 1;
if (nxt == 0)nxt++;
}
rep(i, 1, n + 1) {
if (i == nxt || i == cur || v[i])continue;
if (is_clockwise(cur, nxt, i)) {
nxt = i;
}
}
if (v[nxt])break;
v[nxt] = 1;
cur = nxt;
}
ll ans = 0;
rep(i,0,n)ans += v[i];
give_answer(ans);
return 0;
}
