#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
int main() {
int n = get_n();
vector<int> up, down;
for ( int i = 3; i <= n; i++ ) {
if ( is_clockwise( 1, 2, i ) )
down.push_back( i );
else
up.push_back( i );
}
sort( down.begin(), down.end(), []( int a, int b ) {
return is_clockwise( 1, a, b );
} );
sort( up.begin(), up.end(), []( int a, int b ) {
return is_clockwise( 1, a, b );
} );
vector<int> order;
order.push_back( 1 );
for ( int a: up )
order.push_back( a );
order.push_back( 2 );
for ( int a: down )
order.push_back( a );
vector<int> stack;
stack.push_back( order[0] );
stack.push_back( order[1] );
for ( int i = 2; i < n; i++ ) {
while ( stack.size() >= 2 && !is_clockwise( stack[stack.size() - 2], stack[stack.size() - 1], order[i] ) )
stack.pop_back();
stack.push_back( order[i] );
}
give_answer( stack.size() );
return 0;
}
