#include <bits/stdc++.h>
#include "trilib.h"
using namespace std;
int v[40010] , x[40010] , y[40010] , s[40010];
int cmp (int x , int y){
return !is_clockwise(1 , x , y);
}
int main()
{
int n , i , elem , j , bgn;
n = get_n();
/// stergi citirea, folosesti get_n
for (i = 2 ; i < n ; i++){
if (is_clockwise(n , i , 1))
y[++y[0]] = i;
else x[++x[0]] = i;
}
sort (x + 1 , x + x[0] + 1 , cmp);
sort (y + 1 , y + y[0] + 1 , cmp);
for (i = 1 ; i <= y[0] ; i++)
v[i] = y[i];
v[y[0] + 1] = 1;
for (i = 1 ; i <= x[0] ; i++)
v[y[0] + 1 + i] = x[i];
v[x[0] + y[0] + 2] = n;
s[1] = v[1];
s[2] = v[2];
bgn = 1;
elem = 2;
for (j = 2 ; j <= n ; j++){
while (elem - bgn + 1 >= 2 && is_clockwise (s[elem - 1] , s[elem] , v[j])){
elem--;
}
s[++elem] = v[j];
}
while (elem - bgn + 1 >= 3){
if (is_clockwise (s[elem] , s[bgn] , s[bgn + 1])){
bgn++;
}
else if (is_clockwise (s[elem - 1] , s[elem] , s[bgn]))
elem--;
else break;
}
give_answer(max(3 , elem - bgn + 1) );
return 0;
}
