KHÁC PHÍA

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;

template <class T>
void read(T &x)
{
    x = 0;
    register int c;
    while ((c = getchar()) && (c > '9' || c < '0'))
        ;
    for (; c >= '0' && c <= '9'; c = getchar())
        x = x * 10 + c - '0';
}

constexpr bool typetest = 0;
constexpr int N = 2e5 + 5;
constexpr ll mod = 1e9 + 7;

struct Vector
{
    ll x, y;
    Vector(ll x = 0, ll y = 0) : x(x), y(y) {}
    Vector operator-(const Vector &a) const
    {
        return Vector(x - a.x, y - a.y);
    }

    ll operator*(const Vector &a) const
    {
        return x * a.y - y * a.x;
    }
} d[N];

using Point = Vector;

int n;
ll a, b, c;

void Read()
{
    cin >> n >> a >> b >> c;

    for (int i = 1; i <= n; ++i)
        cin >> d[i].x >> d[i].y;
}

// ax + by = c
// a * x1 + b * y1 = a
// a * x2 + b * y2 = b

pair<ll, ll> Dio(ll a, ll b, ll c)
{
    ll xa = 1, ya = 0, xb = 0, yb = 1, xr, yr, r;

    while (b != 0)
    {
        ll q = a / b;

        r = a - b * q;
        xr = xa - xb * q;
        yr = ya - yb * q;

        a = b;
        xa = xb;
        ya = yb;

        b = r;
        xb = xr;
        yb = yr;
    }

    // d -> gcd(a, b)
    // a * xd + b * yd = gcd(a, b)
    // a * (xd * c/gcd(a, b)) + b * (yd * c/gcd(a, b)) = c;

    ll v = c / a;

    return make_pair(xa * v, ya * v);
}

void Solve()
{
    pair<ll, ll> point = Dio(a, b, -c);

    Point u = Point(point.first, point.second);
    Point v = Point(u.x + b, u.y - a);

    ll cnt[2] = {0, 0};

    for (int i = 1; i <= n; ++i)
        if ((d[i] - u) * (v - d[i]) > 0)
            ++cnt[0];
        else if ((d[i] - u) * (v - d[i]) < 0)
            ++cnt[1];

    cout << cnt[0] * cnt[1];
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    if (fopen("tests.inp", "r"))
    {
        freopen("test.inp", "r", stdin);
        freopen("test.out", "w", stdout);
    }

    int t(1);
    if (typetest)
        cin >> t;

    for (int _ = 1; _ <= t; ++_)
    {
        // cout << "Case #" << _ << endl;
        Read();
        Solve();
    }
    // cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
}

/*

*/

/*
Input:
4 5 4
2 3
2 5
3 1
4 4

Output:
9
*/