#include <stdio.h>
#include "trilib.h"
#include <string.h>
#include <stdlib.h>
int sta[55555], top, n, low[55555], q, w, a[55555], b[55555], c, d, ch_high[55555], ch_low[55555], cha, chb
, sta2[55555], top2;
int cmp_high(const void *ii, const void *jj) {
return is_clockwise(q, *(int*)ii, *(int*)jj) ? -1 : 1;
}
int cmp_low(const void *ii, const void *jj) {
return is_clockwise(q, *(int*)ii, *(int*)jj) ? -1 : 1;
}
int main() {
n = get_n();
q = 1;
w = 2;
for (int i = 3; i <= n; ++i) {
if (is_clockwise(q, w, i)) {
low[i] = 1;
a[c++] = i;
} else {
low[i] = 0;
b[d++] = i;
}
}
qsort(b, d, sizeof *b, cmp_high);
qsort(a, c, sizeof *a, cmp_low);
sta[top = 1] = q;
sta[top = 2] = w;
for (int i = 0; i < d; ++i) {
int j = b[i];
while (top > 2 && is_clockwise(sta[top - 1], sta[top], j))
--top;
sta[++top] = j;
}
cha = top;
memcpy(ch_high, sta, sizeof sta);
sta[top = 1] = w;
sta[top = 2] = q;
for (int i = 0; i < c; ++i) {
int j = a[i];
while (top > 2 && is_clockwise(sta[top - 1], sta[top], j))
--top;
sta[++top] = j;
}
chb = top;
memcpy(ch_low, sta, sizeof sta);
/* stitch around q */
int sta = 0, stb = 0;
++sta, ++stb;
while (1) {
int f = 0;
if (cha - sta + 1 >= 1 && chb - stb + 1 >= 2 && is_clockwise(ch_high[cha ], ch_low[stb + 1], ch_low[stb + 2])) {
++stb;
++f;
}
if (cha - sta + 1 >= 1 && chb - stb + 1 >= 2 && is_clockwise(ch_low[chb - 1], ch_low[chb - 0], ch_high[sta + 1])) {
--chb;
++f;
}
if (chb - stb + 1 >= 1 && cha - sta + 1 >= 2 && is_clockwise(ch_low[chb ], ch_high[sta + 1], ch_high[sta + 2])) {
++sta;
++f;
}
if (chb - stb + 1 >= 1 && cha - sta + 1 >= 2 && is_clockwise(ch_high[cha - 1], ch_high[cha - 0], ch_low[stb + 1])) {
--cha;
++f;
}
if (! f)
break;
}
give_answer(cha + chb - sta - stb);
return 0;
}
