Yandex.Algorithm.2015 Online Round 2.2 Task A

#include <bits/stdc++.h>
using namespace std;

#define FINAL_OUT(x) {cout << x << '\n'; return 0;}

int sign(long long x)
{
    if (x > 0) return 1;
    if (x < 0) return -1;
    return 0;
}

int vector_prod(pair<int,int> a, pair<int,int> b)
{
    return sign(a.first * 1LL * b.second - a.second * 1LL * b.first);
}

int scalar_prod(pair<int,int> a, pair<int,int> b)
{
    return sign(a.first * 1LL * b.first + a.second * 1LL * b.second);
}

int main()
{
    int xs,ys,xt,yt;
    cin >> xs >> ys >> xt >> yt;

    pair<int,int> need(xt - xs, yt - ys);

    if (need == make_pair(0,0))
        FINAL_OUT("YES");

    int n = 0;
    cin >> n;
    vector<pair<int,int> > a(n);
    for(int i = 0; i < n; ++i)
        cin >> a[i].first >> a[i].second;

    for(int i = 0; i < n; ++i)
        for(int j = 0; j < n; ++j)
            if (vector_prod(a[i], need) == 0)
            {
                if (scalar_prod(a[i], need) > 0)
                    FINAL_OUT("YES");
            }
            else
            {
                if (vector_prod(a[i], a[j]) == vector_prod(a[i], need) && vector_prod(a[i], need) == vector_prod(need, a[j]))
                    FINAL_OUT("YES");
            }
    FINAL_OUT("NO");
}