Example geometry

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

const long double PI = acos(-1.0);

struct PT { LL x, y; };

struct V
{
  LL x, y;
  V(LL _x=0, LL _y=0): x(_x), y(_y) { }
  V(PT p): x(p.x), y(p.y) { }
  V(PT p1, PT p2): x(p2.x-p1.x), y(p2.y-p1.y) {}

  LL operator*(V v2)
  {
    return x * v2.x + y * v2.y;
  }

  LL operator%(V v2)
  {
    return x * v2.y - y * v2.x;
  }

  long double len()
  {
    return hypot(x, y); // sqrt(x*x + y*y)
  }
};

long double fi(V v1, V v2)
{
  return atan2(v1 % v2, v1 * v2); // -pi ... pi
}

istream& operator>>(istream& in, PT& p)
{
  in >> p.x >> p.y;
  return in;
}

ostream& operator<<(ostream& out, V v)
{
  out << v.x << ' ' << v.y;
  return out;
}

int main()
{
  #ifdef AZHUKOV
  freopen("input.txt", "r", stdin);
  #endif // AZHUKOV

  PT A, B;
  cin >> A >> B;

  V AB(A,B);
  cout << AB;

  // OA.operator*(OB)
  return 0;
}