convex hull

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cassert>

#define N 1005

using namespace std;

struct vect {
    int x, y;

    void read() {
        cin >> x >> y;
    }

    void print() {
        cout << x << " " << y << endl;
    }

    vect() {}

    vect(int a, int b) {
        x = a, y = b;
    }

    int sq_length() {
        return x * x + y * y;
    }
};

vect operator - (vect a, vect b) {
    return vect(a.x - b.x, a.y - b.y);
}

int operator * (vect a, vect b) {
    return a.x * b.y - a.y * b.x;
}

int operator ^ (vect a, vect b) {
    return a.x * b.x + a.y * b.y;
}

vect start, center, p[N], p2[N], st[N];
int n, top = 0, m;
bool del[N];

void push(vect a) {
    st[top++] = a;
}

bool comp(vect a, vect b) {
    a = a - start;
    b = b - start;
    if (a * b == 0)
        return a.sq_length() < b.sq_length();
    return a * b > 0;
}

int main() {
    cin >> n;
    center.read();

    for (int i = 0; i < n; i++)
        p[i].read();

    start = p[0];
    int ind = 0;
    for (int i = 1; i < n; i++)
        if (p[i].y < start.y || (p[i].y == start.y && p[i].x < start.x))
            start = p[i], ind = i;

    swap(p[0], p[ind]);

    sort(p + 1, p + n, comp);

    for (int i = n - 2; i > 0; i--)
        if ( (p[i] - start) * (p[i + 1] - start) == 0 )
            del[i] = true;

    for (int i = 0; i < n; i++)
        if (!del[i])
            p2[m++] = p[i];

    p2[m++] = p2[0];

    for (int i = 0; i < 3; i++)
        push(p2[i]);

    for (int i = 3; i < m; i++) {
        while ((p2[i] - st[top - 1]) * (st[top - 1] - st[top - 2]) >= 0)
            top--;
        push(p2[i]);
    }

    int answer = 0;

    for (int i = 1; i < top; i++) {
        if ( ((center - st[i - 1]) ^ (st[i] - st[i - 1])) > 0 && ((center - st[i]) ^ (st[i - 1] - st[i])) > 0 )
            answer++;
    }

    cout << answer;

    return 0;
}