ALGO circleGeometry

#include <cmath>
#include <cassert>
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <vector>
#ifdef AC
    #define __GLIBCXX_DEBUG
    #define __GLIBCXX_ASSERT
#endif
using namespace std;

#define all(x) begin(x), end(x)
template<class A> void addlog(A a) { cerr << a << endl; }
template<class A, class... B> void addlog(A a, B... b)
{ cerr << a << ' '; addlog(b...); }


namespace floatGeometry {
    // Types
    #define pcout cout << fixed << setprecision(7)
    typedef long long ll;
    typedef long double ld;
    const ld eps = 1e-12;
    const ld inf = 1e+12;
    bool eq (ld a, ld b) { return abs(a - b) < eps; }
    ld abs (ld x) { return (x > 0? x : -x); }
    struct Point { ld x, y; };
    typedef Point Vector;
    const ld PI = 3.14159265358979L;
    ld toGradus (ld rad) { return (180 * rad) / PI; }
    ld toRad (ld gradus) { return (PI * gradus) / 180; }

    // Constants
    const Point     NO_POINT    {19, -inf},
                    POINTS      {19, +inf};
    // Point
    ld abs (Point p) { return hypot(p.x, p.y); }
    ld ang (Point p) { return atan2(p.y, p.x); }
    bool operator== (Point a, Point b)
    { return eq(a.x, b.x) && eq(a.y, b.y); }
    bool operator!= (Point a, Point b)
    { return !(a == b); }
    // Vector
    Vector operator- (Vector v)
    { return Vector{ -v.x, -v.y }; }
    Vector operator+ (Vector a, Vector b)
    { return Vector{ a.x+b.x, a.y+b.y }; }
    Vector operator- (Vector a, Vector b)
    { return Vector{ a.x-b.x, a.y-b.y }; }
    Vector operator* (Vector v, ld alpha)
    { return Vector{ alpha * v.x, alpha * v.y }; }
    ld operator* (Vector a, Vector b)
    { return a.x*b.y - b.x*a.y; }
    ld operator% (Vector a, Vector b)
    { return a.x*b.x + a.y*b.y; }
    Vector turn90 (Vector v)
    { return Vector{v.y, -v.x}; }
    ld ang (Vector a, Vector b) {
        return atan2(a * b, a % b);
    }
    // I/O.
    istream& operator>> (istream& s, Point &p)
    { return s >> p.x >> p.y; }
    ostream& operator<< (ostream &s, Point p)
    { return s << p.x << ' ' << p.y; }


    struct Segment { Point p, q; };
    ld abs (Segment s) { return abs(s.p - s.q); }

    istream& operator>> (istream &s, Segment &x)
    { return s >> x.p >> x.q; }
    ostream& operator<< (ostream &s, Segment x)
    { return s << x.p << "  " << x.q; }

    struct Line { ld a, b, c; Line () {}
        void normalize () {
            ld z = hypot(a, b);
            if (eq(a, 0)) { if (b < 0) z = -z; }
            else if (a < 0) z = -z;
            a /= z; b /= z; c /= z;
                                                                        assert(eq(hypot(a, b), 1));
        } Line (Point p, Point q) {
            a = p.y - q.y; b = q.x - p.x;
            c = - (a * p.x + b * p.y );
            normalize();
        } Line (Segment s) : Line(s.p, s.q) {}
    };

    // Line
    ld vMul (ld ax, ld ay, ld bx, ld by) { return ax*by - bx*ay; }
    Point intersect (Line I, Line II) {
        ld D0 = vMul(I.a, I.b, II.a, II.b);
        ld Dx = vMul(I.b, I.c, II.b, II.c);
        ld Dy = vMul(I.c, I.a, II.c, II.a);
        if (eq(D0, 0)) {
            assert(eq(Dx, Dy));
            if (eq(Dx, 0))  return POINTS;
            else            return NO_POINT;
        } return Point{Dx / D0, Dy / D0};
    }
    // I/O
    istream& operator>> (istream& s, Line &l)
    { return s >> l.a >> l.b >> l.c; }
    ostream& operator<< (ostream& s, Line l)
    { return s << l.a << ' ' << l.b << ' ' << l.c; }

    struct Ray {
        Point p; Vector v; Line l;
        Ray (Point P, Vector V) : p(P), v(V), l(p, p + v) {}
    };

    ld signDist (Line l, Point p) {
        l.normalize();
        return l.a * p.x + l.b * p.y + l.c;
    } ld dist (Line l, Point p) { return abs(signDist(l, p)); }

    ld dist (Ray r, Point p) {
        if ( (p - r.p) % r.v > -eps )
            return dist(r.l, p);
        else return abs(p - r.p);
    }

    ld dist (Segment s, Point p) {
        Ray r1 (s.p, s.q - s.p),
            r2 (s.q, s.p - s.q);
        return max(dist(r1, p), dist(r2, p));
    }

    template <class G> bool
    has (G g, Point p)
    { return eq(dist(g, p), 0); }

    Point intersect (Segment a, Segment b) {
        Line la(a), lb(b);
        Point p = intersect(la, lb);
        if (p == POINTS) {
            // Return one of that points.
            if (has(a, b.p)) return b.p;
            if (has(a, b.q)) return b.q;
            if (has(b, a.p)) return a.p;
            if (has(b, a.q)) return a.q;
                                                                        assert(false);
        } else if (p != NO_POINT && has(a, p) && has(b, p)) {
            return p;
        } else
            return NO_POINT;
    }
}


namespace circleGeometry {
    using namespace floatGeometry;
    typedef pair<Point, Point> twoPoints;
    struct Circle { Point O; ld r; };

    istream &operator>> (istream &s, Circle &c)
    { return s >> c.O >> c.r; }
    ostream &operator<< (ostream &s, Circle c)
    { return s << c.O << "  " << c.r; }

    twoPoints intersect (Circle c, Line l) {
        ld d = dist(l, c.O);                                            //addlog("dist =", d);
        if (c.r < d - eps) return {NO_POINT, NO_POINT};
        ld L = sqrt( c.r * c.r - d * d );
        Vector perL{ l.a, l.b };
        Vector OH = perL * (d / abs(perL));
        Point H = c.O + OH; if (!has(l, H)) H = c.O - OH;               assert(has(l, H)); // ????
        Vector parL = turn90(perL);
        Vector dH = parL * (L / abs(parL));
        twoPoints res = {H + dH, H - dH};
        if (eq(c.r, d)) {
            assert(res.first == res.second);
            res.first = NO_POINT;
        } return res;
    }

    Point _check (Point p, Segment s) {
        if (p == NO_POINT || p == POINTS || has(s, p)) return p;
        return NO_POINT;
    }

    twoPoints intersect (Circle c, Segment s) {
        Line l(s);
        twoPoints tp = intersect(c, l);
        twoPoints res{ _check(tp.first, s), _check(tp.second, s) };
        if (res.second == NO_POINT) swap(res.first, res.second);
        return res;
    }
}




using namespace circleGeometry;
int main () {

    for (Vector a, b; cin >> a >> b; )
        pcout << ang(a, b) << endl;

    return 0;
}