#include <bits/stdc++.h>
#include <iostream>
#include "trilib.h"
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef tuple<int,int,int> tiii;
#define rep(i,a,b) for(int i = a; i < b; i++)
#define pb push_back
int n, guesses = 1e6;
map<tiii, bool> cw;
bool clockwise(int a, int b, int c){
tiii t = make_tuple(a, b, c);
if (cw.find(t) != cw.end()) return cw[t];
bool ans = is_clockwise(a, b, c);
cw[{a, b, c}] = cw[{b, c, a}] = cw[{c, a, b}] = ans;
cw[{a, c, b}] = cw[{c, b, a}] = cw[{b, a, c}] = !ans;
return ans;
}
bool hull(int a, int b) {
rep(c, 1, n + 1) {
if (c == a || c == b) continue;
if (!clockwise(a, b, c)) return false;
}
return true;
}
int find_neighbor(int a) {
//a down, b up
int b = (a != 1) ? 1:2;
rep(c, 1, n+1) {
if (c == a || c == b) continue;
if (clockwise(a, c, b)) b = c;
}
return b;
}
int main()
{
n = get_n();
set<int> h;
rep(i, 1, n + 1) {
int j = find_neighbor(i);
if (hull(i, j) || hull(j, i)) {h.insert(i); h.insert(j);}
}
cout << h.size() << endl;
}
