This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
#define in(x) freopen(x, "r", stdin)
#define out(x) freopen(x, "w", stdout)
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC optimize("-O3")
#define F first
#define S second
#define PB push_back
#define M ll(1e9 + 7)
#define sz(x) int(x.size())
#define N 1001
#define pri(x) cout << x << endl
#define endl '\n'
#define all(x) (x).begin(), (x).end()
#define _ << " " <<
using namespace std;
//typedef tree <ll, null_type, less_equal <ll> , rb_tree_tag, tree_order_statistics_node_update> ordered_set;
//using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
typedef short int si;
typedef unsigned long long ull;
const ld eps = 1e-10;
pair <ld, ld> convert(int x1, int y1, int x2, int y2)
{
ld k = 0;
if (x2 != x1)
k = ld(y2 - y1) / ld(x2 - x1);
ld b = ld(y1) - k * x1;
return {k, b};
}
pair <ld, ld> inter(pair <ld, ld> x, pair <ld, ld> y)
{
if (fabs(x.F - y.F) < eps)
return {1e18, 1e18};
ld X = (y.S - x.S) / (x.F - y.F);
ld Y = (x.F * X + x.S);
return {X, Y};
}
pair <ld, ld> otr[N][N];
int main()
{
istream::sync_with_stdio(0); ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
// freopen("1.in", "r", stdin);
int n;
cin >> n;
int x[n], y[n];
for (int i = 0; i < n; i++)
cin >> x[i] >> y[i];
for (int i = 0; i < n; i++)
for (int j = i + 1; j < n; j++)
otr[i][j] = convert(x[i], y[i], x[j], y[j]);
int ans = 0;
for (int i = 0; i < n; i++)
for (int j = i + 1; j < n; j++)
{
bool gd = 0;
for (int u = 0; u < n && !gd; u++)
{
if (i == u || j == u)
continue;
for (int t = u + 1; t < n && !gd; t++)
{
if (t == i || t == j)
continue;
if (x[i] == x[j])
{
if (x[u] == x[t])
{
continue;
}
else if (y[u] == y[t])
{
if (min(x[u], x[t]) <= x[i] && x[i] <= max(x[u], x[t]) && min(y[i], y[j]) <= y[u] && y[u] <= max(y[i], y[j]))
{
gd = 1;
}
continue;
}
else
{
if (min(x[u], x[t]) <= x[i] && x[i] <= max(x[u], x[t]))
{
ld yy = otr[u][t].F * x[i] + otr[u][t].S;
if (min(y[i], y[j]) - yy < eps && yy - max(y[i], y[j]) < eps)
{
gd = 1;
}
}
continue;
}
}
if (y[i] == y[j])
{
if (y[u] == y[t])
{
continue;
}
else if (x[u] == x[t])
{
if (min(x[i], x[j]) <= x[u] && x[u] <= max(x[i], x[j]) && min(y[u], y[t]) <= y[i] && y[i] <= max(y[u], y[t]))
{
gd = 1;
}
continue;
}
else
{
if (min(y[u], y[t]) <= y[i] && y[i] <= max(y[u], y[t]))
{
ld xx = (y[i] - otr[u][t].S) / otr[u][t].F;
if (min(x[i], x[j]) - xx < eps && xx - max(x[i], x[j]) < eps)
{
gd = 1;
}
}
continue;
}
}
if (x[u] == x[t])
{
if (min(x[i], x[j]) <= x[u] && x[u] <= max(x[i], x[j]))
{
ld yy = otr[i][j].F * x[u] + otr[i][j].S;
if (min(y[u], y[t]) - yy < eps && yy - max(y[u], y[t]) < eps)
{
gd = 1;
}
}
continue;
}
if (y[u] == y[t])
{
if (min(y[i], y[j]) <= y[u] && y[u] <= max(y[i], y[j]))
{
ld xx = (y[u] - otr[i][j].S) / otr[i][j].F;
if (min(x[u], x[t]) - xx < eps && xx - max(x[u], x[t]) < eps)
{
gd = 1;
}
}
continue;
}
pair <ld, ld> cur = inter(otr[i][j], otr[u][t]);
ld lx = min(x[i], x[j]);
ld rx = max(x[i], x[j]);
ld ly = min(y[i], y[j]);
ld ry = max(y[i], y[j]);
if (lx - cur.F < eps && cur.F - rx < eps)
{
if (ly - cur.S < eps && cur.S - ry < eps)
{
ld lx = min(x[u], x[t]);
ld rx = max(x[u], x[t]);
ld ly = min(y[u], y[t]);
ld ry = max(y[u], y[t]);
if (lx - cur.F < eps && cur.F - rx < eps)
{
if (ly - cur.S < eps && cur.S - ry < eps)
{
gd = 1;
}
}
}
}
}
}
if (!gd)
ans++;
}
pri(ans);
}
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