H

#include <iostream>
#include <cmath>
#include <iomanip>

using namespace std;

struct Point {
    double x, y;

    Point(double x0 = 0, double y0 = 0)
    {
        x = x0;
        y = y0;
    }
};

istream & operator >> (istream & in, Point & P) {
    in >> P.x >> P.y;
    return in;
}


struct Vector {
    double x, y;

    Vector(double x0, double y0) {
        x = x0;
        y = y0;
    }

    Vector(Point A, Point B) {
        x = B.x - A.x;
        y = B.y - A.y;
    }
};


double scalarMulripl(Vector a, Vector b) {
    return a.x * b.x + a.y * b.y;
}


double vectMultipl(Vector a, Vector b) {
    return a.x * b.y - b.x * a.y;
}


int main() {
    Point O, A, B;
    double r;
    cin >> O >> r >> A >> B;
    cout << fixed << setprecision(10);
    if (r == 0) {
        cout << 0;
        return 0;
    }
    Vector OA = Vector(O, A);
    Vector OB = Vector(O, B);
    double sinAlpha = vectMultipl(OA, OB) / (r * r);
    double cosAlpha = scalarMulripl(OA, OB) / (r * r);
    double alpha = atan2(sinAlpha, cosAlpha);
    cout << abs(r * alpha);
    return 0;
}