#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define sz(x) (int)x.size()
static int N;
static long long *x, *y;
static int queries=0;
static void init() {
static int is_inited=0;
if (is_inited)
return;
is_inited=1;
assert(scanf("%d", &N)==1);
x=(long long*)malloc((N+1)*sizeof(long long));
y=(long long*)malloc((N+1)*sizeof(long long));
for (int i=1; i<=N; i++)
assert(scanf("%lld%lld", &x[i], &y[i])==2);
}
int get_n() {
init();
return N;
}
int is_clockwise(int a, int b, int c) {
init();
assert(a>=1 && a<=N);
assert(b>=1 && b<=N);
assert(c>=1 && c<=N);
assert(a!=b && a!=c && b!=c);
queries++;
if(queries == 1000 * 1000 + 1)
printf("Too many queries!");
return (x[b]-x[a])*(y[c]-y[a])-(x[c]-x[a])*(y[b]-y[a])<0;
}
void give_answer(int s) {
init();
printf("%d\n", s);
}
int n, cnt = 0, st;
vector<int> v, stk1, stk2;
int main()
{
n = get_n();
v.pb(1);
v.pb(2);
for(int i = 3; i <= n; i++)
{
while(sz(v) > 1 && is_clockwise(v[sz(v) - 2], v[sz(v) - 1], i))
v.pop_back();
v.pb(i);
}
cnt = 1;
st = v[1];
int lst = v[1];
while(1)
{
for(int i = 1; i <= n; i++)
{
if(i == lst) continue;
bool ok = true;
for(int j = 1; j <= n; j++)
{
if(j == i || j == lst) continue;
if(!is_clockwise(lst, i, j))
{
ok = false;
break;
}
}
if(ok)
{
// printf("%d %d\n",lst,i);
if(i == st)
{
give_answer(cnt);
return 0;
}
cnt++;
lst = i;
break;
}
}
}
return 0;
}
