#include <iostream>
#include <fstream>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <cstdlib>
#include <cstdio>
#include <string>
#include <cstring>
#include <cassert>
#include <utility>
#include <iomanip>
#define sqr(x) ((x)*(x))

using namespace std;

const double eps = 1e-9;

double xc, yc, zc, r;
double xa, ya, za;
double xb, yb, zb;
double a, b, c;
double cosA, cosB, cosC;// angles
double sinA;
double mysqrt(double x) {
    if (x < 0.0 + eps) x = 0.0;
    return sqrt(x);
}
double myacos(double x) {
    if (x < -1.0 + eps)
        x = -1.0;
    else if (x + eps > 1.0)
        x = 1.0;
    return acos(x);
}

int main() {
    assert(freopen("input.txt","r",stdin));
    assert(freopen("output.txt","w",stdout));
    scanf("%lf%lf%lf%lf", &xc, &yc, &zc, &r);
    scanf("%lf%lf%lf", &xa, &ya, &za);
    scanf("%lf%lf%lf", &xb, &yb, &zb);
    b = sqrt( sqr(xa-xc)+sqr(ya-yc) + sqr(za-zc));
    c = sqrt( sqr(xa-xb) + sqr(ya-yb) + sqr(za -zb) );
    a = sqrt( sqr(xc-xb) + sqr(yc-yb) + sqr(zc -zb) );
    double beta = asin(r/b);
    cosA = (b*b + c*c - a*a) / (2*b*c);
    if ( b + c <= a + eps || c + a <= b + eps ) {
        printf("%.12lf\n", 0);
        return 0;
    }


    double lo = 0, hi = c, x;
    double CO, cosTh, p, cosPhi, q, f;
    int step = 1000;
    while ( step -- ) {
        x = (hi + lo ) / 2;
        CO = mysqrt(x*x + b*b - 2*x*b*cosA);
        cosTh = (b*b + CO*CO - x*x) / (2 * b * CO);
        p = mysqrt(b*b + r*r - 2*b*r*cosTh);
        cosPhi = (CO*CO + a*a - sqr(c - x)) / (2*CO*a);
        q = mysqrt( r*r + a*a - 2*r*a*cosPhi);
        f = p * ( c - x ) / (q*x);
        if (f < 1.0)
            hi = x;
        else
            lo = x;
    }
    double alpha = myacos( (p*p + b*b - r*r) / (2 * p * b) );
    double diff = fabs(p * (c-x) - q*x);
    //printf("diff = %.15lf\n", diff);

    //assert( fabs(p * (c-x) - q*x) < 1e-9);
    printf("%.12lf\n", alpha/beta);

    return 0;
}