#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
// macros
typedef long long ll;
typedef long double ld;
typedef pair<int, int> ii;
typedef pair<ll, ll> lll;
typedef tuple<int, int, int> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef vector<iii> viii;
typedef vector<ll> vll;
typedef vector<lll> vlll;
#define REP(a,b,c) for(int a=int(b); a<int(c); a++)
#define RE(a,c) REP(a,0,c)
#define RE1(a,c) REP(a,1,c+1)
#define REI(a,b,c) REP(a,b,c+1)
#define REV(a,b,c) for(int a=int(c-1); a>=int(b); a--)
#define FOR(a,b) for(auto& a : b)
#define all(a) a.begin(), a.end()
#define INF 1e9
#define EPS 1e-9
#define pb push_back
#define popb pop_back
#define fi first
#define se second
#define sz size()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
// input
template<class T> void IN(T& x) {cin >> x;}
template<class H, class... T> void IN(H& h, T&... t) {IN(h); IN(t...); }
// output
template<class T1, class T2> void OUT(const pair<T1,T2>& x);
template<class T> void OUT(const T& x) {cout << x;}
template<class H, class... T> void OUT(const H& h, const T&... t) {OUT(h); OUT(t...); }
template<class T1, class T2> void OUT(const pair<T1,T2>& x) {OUT(x.fi," ",x.se);}
template<class... T> void OUTL(const T&... t) {OUT(t..., "\n"); }
//===================//
// Added libraries //
//===================//
//===================//
//end added libraries//
//===================//
void program();
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
program();
}
//===================//
// begin program //
//===================//
const int MX = 50000;
int n, m;
int SHUF[MX];
bool cw(int x, int y, int z) {
return is_clockwise(SHUF[x],SHUF[y],SHUF[z]);
}
bool ccw(int x, int y, int z) {
return !is_clockwise(SHUF[x],SHUF[y],SHUF[z]);
}
int MEX(int x, int y) {
RE1(i,3)
if(x != i && y != i) return i;
}
int origin;
int S[2][MX], C[2][MX], N[2], M[2];
set<int> onSide[2];
int ST[2][MX], O[2];
int s1, s2;
void program() {
n = get_n();
RE1(i,n) SHUF[i] = i;
shuffle(SHUF+1,SHUF+n+1,rng);
if(n == 3) {
give_answer(3);
return;
}
// create two halves
s1 = 1;
s2 = 2;
REI(j,3,n) onSide[cw(s1,s2,j)].insert(j);
RE(i,2) {
// i == 0 left side of 1->2, i == 1 right side of 1->2
onSide[i].insert(s1);
onSide[i].insert(s2);
origin = i+1;
N[i] = 0;
FOR(j,onSide[i]) S[i][N[i]++] = j;
RE(j,N[i]) if(S[i][j] == origin) swap(S[i][j], S[i][0]);
sort(S[i]+1,S[i]+N[i],[](int i, int j) {
return ccw(origin, i, j);
});
M[i]=0;
C[i][M[i]++] = S[i][N[i]-1], C[i][M[i]++] = S[i][0];
int j=1;
if(N[i] != 2) {
while(j < N[i]) {
if(ccw(C[i][M[i]-2],C[i][M[i]-1],S[i][j])) C[i][M[i]++] = S[i][j++];
else M[i]--;
}
} else {
C[i][M[i]++] = S[i][1];
}
}
// merge two halves
int answer = M[0]+M[1]-2-2;
RE(h,2) {
RE(i,2) O[i]=0;
REP(j,2,M[ h]) ST[ h][O[ h]++] = C[ h][j];
REP(j,2,M[!h]) ST[!h][O[!h]++] = C[!h][j];
reverse(ST[h], ST[h]+O[h]);
int c[2];
RE(i,2) c[i]=0;
while((O[h]-c[h]) && (O[!h]-c[!h])) {
if(O[h]-c[h] >= 2) {
if(cw(ST[h][c[h]+1],ST[h][c[h]],ST[!h][c[!h]])) {
answer--;
c[h]++;
continue;
}
}
if(O[!h]-c[!h] >= 2) {
if(ccw(ST[!h][c[!h]+1],ST[!h][c[!h]],ST[h][c[h]])) {
answer--;
c[!h]++;
continue;
}
}
break;
}
}
give_answer(answer);
}
