Problem p01204 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

#define REP(i, s, n) for (int i = s; i < n; i++)
#define rep(i, n) REP(i, 0, n)
#define IINF (INT_MAX)
#define EPS (1e-7)
#define equals(a, b) (fabs((a) - (b)) < EPS)

using namespace std;

// 作成中 VerifyしたものにはVerifyと書いてある

// Verify AOJ 0115
class Point3d {
public:
  double x, y, z;

  Point3d(double x = 0, double y = 0, double z = 0) : x(x), y(y), z(z) {}

  Point3d operator+(const Point3d &a) {
    return Point3d(x + a.x, y + a.y, z + a.z);
  }
  Point3d operator-(const Point3d &a) {
    return Point3d(x - a.x, y - a.y, z - a.z);
  }
  Point3d operator*(const double &d) { return Point3d(x * d, y * d, z * d); }
  Point3d operator/(const double &d) { return Point3d(x / d, y / d, z / d); }

  bool operator<(const Point3d &p) const {
    return !equals(x, p.x) ? x < p.x : ((!equals(y, p.y)) ? y < p.y : z < p.z);
  }

  bool operator==(const Point3d &p) const {
    return equals(x, p.x) && equals(y, p.y) && equals(z, p.z);
  }
};

// Verify AOJ 0115
struct Segment3d {
  Point3d p[2];
  Segment3d(Point3d p1 = Point3d(), Point3d p2 = Point3d()) {
    p[0] = p1, p[1] = p2;
  }
  bool operator==(const Segment3d &seg) const {
    return p[0] == seg.p[0] && p[1] == seg.p[1];
  }
};

typedef Point3d Vector3d;
typedef Segment3d Line3d;

ostream &operator<<(ostream &os, const Point3d &p) {
  os << "(" << p.x << "," << p.y << "," << p.z << ")";
}

ostream &operator<<(ostream &os, const Segment3d &seg) {
  os << "(" << seg.p[0] << "," << seg.p[1] << ")";
}

// Verify AOJ 0115
double dot(const Point3d &a, const Point3d &b) {
  return a.x * b.x + a.y * b.y + a.z * b.z;
}

// Verify AOJ 0115
Vector3d cross(const Point3d &a, const Point3d &b) {
  return Vector3d(a.y * b.z - a.z * b.y, a.z * b.x - a.x * b.z,
                  a.x * b.y - a.y * b.x);
}

// Verify AOJ 0115
inline double norm(const Point3d &p) {
  return p.x * p.x + p.y * p.y + p.z * p.z;
}

// Verify AOJ 0115
inline double abs(const Point3d &p) { return sqrt(norm(p)); }

inline double toRad(double theta) { return theta * M_PI / 180.0; }

bool on_line3d(Line3d line, Point3d p) {
  return equals(abs(cross(p - line.p[0], line.p[1] - line.p[0])), 0);
}

bool on_segment3d(Segment3d seg, Point3d p) {
  if (!on_line3d(seg, p))
    return false;
  double dist[3] = {abs(seg.p[1] - seg.p[0]), abs(p - seg.p[0]),
                    abs(p - seg.p[1])};
  return on_line3d(seg, p) && equals(dist[0], dist[1] + dist[2]);
}

// Verify AOJ 0115
bool point_on_the_triangle3d(Point3d tri1, Point3d tri2, Point3d tri3,
                             Point3d p) {

  // 線分上にpがあった場合、三角形内とみなす
  // if( on_segment3d(Segment3d(tri1,tri2),p) ) return true;
  // if( on_segment3d(Segment3d(tri2,tri3),p) ) return true;
  // if( on_segment3d(Segment3d(tri3,tri1),p) ) return true;

  Vector3d v1 = tri2 - tri1;
  Vector3d v2 = tri3 - tri2;
  Vector3d v3 = tri1 - tri3;

  Vector3d cp[3] = {cross(v1, p - tri1), cross(v2, p - tri2),
                    cross(v3, p - tri3)};
  double d1 = dot(cp[0], cp[1]);
  double d2 = dot(cp[0], cp[2]);

  // 線分上にpがあった場合,三角形内とはみなさない
  // if( ( !equals(d1,0.0) && d1 > 0 ) && ( !equals(d2,0.0) && d2 > 0 ) ) return
  // true;

  // 線分上にpがあった場合,三角形内とみなす
  if ((equals(d1, 0.0) || d1 > 0) && (equals(d2, 0.0) || d2 > 0))
    return true;
  return false;
}

inline Point3d rotateX(Point3d p, double rad) {
  return Point3d(p.x, p.y * cos(rad) - p.z * sin(rad),
                 p.y * sin(rad) + p.z * cos(rad));
}

inline Point3d rorateY(Point3d p, double rad) {
  return Point3d(p.x * cos(rad) + p.z * sin(rad), p.y,
                 -p.x * sin(rad) + p.z * cos(rad));
}

inline Point3d rorateZ(Point3d p, double rad) {
  return Point3d(p.x * cos(rad) - p.y * sin(rad),
                 p.x * sin(rad) + p.y * cos(rad), p.z);
}

inline Point3d rotateEuler(Point3d p, double alpha, double beta, double gamma) {
  return Point3d(
      (cos(alpha) * cos(beta) * cos(gamma) - sin(alpha) * sin(gamma)) * p.x +
          (-cos(alpha) * cos(beta) * sin(gamma) - sin(alpha) * cos(gamma)) *
              p.y +
          (cos(alpha) * sin(beta)) * p.z,
      (sin(alpha) * cos(beta) * cos(gamma) + cos(alpha) * sin(gamma)) * p.x +
          (-sin(alpha) * cos(beta) * sin(gamma) + cos(alpha) * cos(gamma)) *
              p.y +
          (sin(alpha) * sin(beta)) * p.z,
      (-sin(beta) * cos(gamma)) * p.x + (sin(beta) * sin(gamma)) * p.y +
          (cos(beta)) * p.z);
}

inline Point3d rotateRollPitchYaw(Point3d p, double roll, double pitch,
                                  double yaw) {
  return Point3d(
      (cos(roll) * cos(pitch)) * p.x +
          (cos(roll) * sin(pitch) * sin(yaw) - sin(roll) * cos(yaw)) * p.y +
          (cos(roll) * sin(pitch) * cos(yaw) + sin(roll) * sin(yaw)) * p.z,
      (sin(roll) * cos(pitch)) * p.x +
          (sin(roll) * sin(pitch) * sin(yaw) + cos(roll) * cos(yaw)) * p.y +
          (sin(roll) * sin(pitch) * cos(yaw) - cos(roll) * sin(yaw)) * p.z,
      -sin(pitch) * p.x + cos(pitch) * sin(yaw) * p.y +
          cos(pitch) * cos(yaw) * p.z);
}

class Plane3d {
public:
  Point3d normal_vector; // 法線ベクトル
  double d; // 平面方程式 normal_vector.x * x + normal_vector.y * y +
            // normal_vector.z * z + d = 0

  Plane3d(Point3d normal_vector = Point3d(), double d = 0)
      : normal_vector(normal_vector), d(d) {}
  Plane3d(Vector3d a, Vector3d b, Vector3d c) {
    Vector3d v1 = b - a;
    Vector3d v2 = c - a;
    Vector3d tmp = cross(v1, v2);
    normal_vector = tmp / abs(tmp);
    set_d(a);
  }

  // Verify AOJ 0115
  // 法線ベクトルnormal_vectorと平面上の１点からdを計算する
  void set_d(Point3d p) { d = dot(normal_vector, p); }

  // 平面と点pの距離を求める
  double distanceP(Point3d p) {
    Point3d a = normal_vector * d; // 平面上の適当な点をつくる
    return abs(dot(p - a, normal_vector));
  }

  // 平面上でもっとも点pと近い点を求める
  Point3d nearest_point(Point3d p) {
    Point3d a = normal_vector * d;
    return p - (normal_vector * dot(p - a, normal_vector));
  }

  // Verify AOJ 0115
  // 平面と線分が交差するか
  bool intersectS(Segment3d seg) {
    Point3d a = normal_vector * d;
    double res1 = dot(a - seg.p[0], normal_vector);
    double res2 = dot(a - seg.p[1], normal_vector);
    if (res1 > res2)
      swap(res1, res2);
    // cout << res1 << " < " << res2 << endl;
    if ((equals(res1, 0.0) || res1 < 0) && (equals(res2, 0.0) || res2 > 0))
      return true;
    return false;
  }

  // Verify AOJ 0115
  // 平面と線分の交点を求める
  Point3d crosspointS(Segment3d seg) {
    Point3d a = normal_vector * d;
    double dot_p0a = fabs(dot(seg.p[0] - a, normal_vector));
    double dot_p1a = fabs(dot(seg.p[1] - a, normal_vector));
    if (equals(dot_p0a + dot_p1a, 0))
      return seg.p[0];
    return seg.p[0] + (seg.p[1] - seg.p[0]) * (dot_p0a / (dot_p0a + dot_p1a));
  }
};

double distanceLP(Line3d line, Point3d p) {
  return abs(cross(line.p[1] - line.p[0], p - line.p[0])) /
         abs(line.p[1] - line.p[0]);
}

Point3d project(Segment3d seg, Point3d p) {
  Vector3d base = seg.p[1] - seg.p[0];
  double t = dot(p - seg.p[0], base) / norm(base);
  return seg.p[0] + base * t;
}

Point3d reflect(Segment3d seg, Point3d p) {
  return p + (project(seg, p) - p) * 2.0;
}

// Verify AOJ 2081
bool isParallel(Line3d l1, Line3d l2) {
  Vector3d A = l1.p[0], B = l1.p[1], C = l2.p[0], D = l2.p[1];
  Vector3d AB = B - A, CD = D - C;
  Vector3d n1 = AB / abs(AB), n2 = CD / abs(CD);
  double tmp = dot(n1, n2);
  tmp = 1 - tmp * tmp;
  return equals(tmp, 0.0);
}

// Verify AOJ 2081
// l1,l2が平行な時には使用できないので注意
Segment3d nearest_segmentLL(Line3d l1, Line3d l2) {
  assert(isParallel(l1, l2)); // 平行な場合は使用不可
  // l1.p[0] = A, l1.p[1] = B, l2.p[0] = C, l2.p[1] = D
  Vector3d AB = l1.p[1] - l1.p[0];
  Vector3d CD = l2.p[1] - l2.p[0];
  Vector3d AC = l2.p[0] - l1.p[0];
  Vector3d n1 = AB / abs(AB), n2 = CD / abs(CD);
  double d1 =
      (dot(n1, AC) - dot(n1, n2) * dot(n2, AC)) / (1.0 - pow(dot(n1, n2), 2));
  double d2 =
      (dot(n1, n2) * dot(n1, AC) - dot(n2, AC)) / (1.0 - pow(dot(n1, n2), 2));
  return Segment3d(l1.p[0] + n1 * d1, l2.p[0] + n2 * d2);
}

// Verify AOJ 2081
bool intersectLL(Line3d l1, Line3d l2) {
  Vector3d A = l1.p[0], B = l1.p[1], C = l2.p[0], D = l2.p[1];

  // そもそもl1,l2が直線じゃない
  if (equals(abs(B - A), 0.0) || equals(abs(D - C), 0.0)) {
    /*
      この場合は注意
      そもそも与えられた線分が線分になっていないので、交差するかどうかは判定できない
    */
    return false;
  }

  Vector3d AB = B - A, CD = D - C;
  Vector3d n1 = AB / abs(AB), n2 = CD / abs(CD);
  double tmp = dot(n1, n2);
  tmp = 1 - tmp * tmp;

  if (equals(tmp, 0.0))
    return 0; // 直線が平行

  Segment3d ns = nearest_segmentLL(l1, l2);
  if (ns.p[0] == ns.p[1])
    return true;
  return false;
}

bool intersectSS(Segment3d seg1, Segment3d seg2) {
  if (isParallel(seg1, seg2))
    return false;
  Segment3d seg = nearest_segmentLL(seg1, seg2);
  if (!(seg.p[0] == seg.p[1]))
    return false;
  Point3d cp = seg.p[1];
  return on_segment3d(seg1, cp) && on_segment3d(seg2, cp);
}

Line3d line[110];
Point3d source_p, sink_p;
int source, sink;
double fw[110][110];

int main() {
  int N;
  while (cin >> N, N) {
    cin >> source_p.x >> source_p.y >> source_p.z >> sink_p.x >> sink_p.y >>
        sink_p.z;

    source = sink = -1;
    rep(i, N) {
      rep(j, 2) cin >> line[i].p[j].x >> line[i].p[j].y >> line[i].p[j].z;
      if (on_line3d(line[i], source_p)) {
        assert(source == -1);
        source = i;
      }
      if (on_line3d(line[i], sink_p)) {
        assert(sink == -1);
        sink = i;
      }
    }
    assert(source != -1 && sink != -1);

    rep(i, N) rep(j, N) fw[i][j] = IINF;
    rep(i, N) REP(j, i, N) {
      if (i == j)
        fw[i][j] = 0;
      else if (isParallel(line[i], line[j])) {
        double area = abs(
            cross(line[i].p[1] - line[i].p[0], line[j].p[1] - line[i].p[0]));
        fw[i][j] = fw[j][i] = area / abs(line[i].p[1] - line[i].p[0]);
      } else if (!intersectLL(line[i], line[j])) {
        Segment3d seg = nearest_segmentLL(line[i], line[j]);
        fw[i][j] = fw[j][i] = abs(seg.p[1] - seg.p[0]);
      }
    }

    rep(k, N) rep(i, N)
        rep(j, N) if (!equals(fw[i][k], IINF) && !equals(fw[k][j], IINF))
            fw[i][j] = min(fw[i][j], fw[i][k] + fw[k][j]);
    printf("%.4f\n", fw[source][sink]);
  }
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>

#define REP(i, s, n) for (int i = s; i < n; i++)
#define rep(i, n) REP(i, 0, n)
#define IINF (INT_MAX)
#define EPS (1e-7)
#define equals(a, b) (fabs((a) - (b)) < EPS)

using namespace std;

// 作成中 VerifyしたものにはVerifyと書いてある

// Verify AOJ 0115
class Point3d {
public:
  double x, y, z;

  Point3d(double x = 0, double y = 0, double z = 0) : x(x), y(y), z(z) {}

  Point3d operator+(const Point3d &a) {
    return Point3d(x + a.x, y + a.y, z + a.z);
  }
  Point3d operator-(const Point3d &a) {
    return Point3d(x - a.x, y - a.y, z - a.z);
  }
  Point3d operator*(const double &d) { return Point3d(x * d, y * d, z * d); }
  Point3d operator/(const double &d) { return Point3d(x / d, y / d, z / d); }

  bool operator<(const Point3d &p) const {
    return !equals(x, p.x) ? x < p.x : ((!equals(y, p.y)) ? y < p.y : z < p.z);
  }

  bool operator==(const Point3d &p) const {
    return equals(x, p.x) && equals(y, p.y) && equals(z, p.z);
  }
};

// Verify AOJ 0115
struct Segment3d {
  Point3d p[2];
  Segment3d(Point3d p1 = Point3d(), Point3d p2 = Point3d()) {
    p[0] = p1, p[1] = p2;
  }
  bool operator==(const Segment3d &seg) const {
    return p[0] == seg.p[0] && p[1] == seg.p[1];
  }
};

typedef Point3d Vector3d;
typedef Segment3d Line3d;

ostream &operator<<(ostream &os, const Point3d &p) {
  os << "(" << p.x << "," << p.y << "," << p.z << ")";
}

ostream &operator<<(ostream &os, const Segment3d &seg) {
  os << "(" << seg.p[0] << "," << seg.p[1] << ")";
}

// Verify AOJ 0115
double dot(const Point3d &a, const Point3d &b) {
  return a.x * b.x + a.y * b.y + a.z * b.z;
}

// Verify AOJ 0115
Vector3d cross(const Point3d &a, const Point3d &b) {
  return Vector3d(a.y * b.z - a.z * b.y, a.z * b.x - a.x * b.z,
                  a.x * b.y - a.y * b.x);
}

// Verify AOJ 0115
inline double norm(const Point3d &p) {
  return p.x * p.x + p.y * p.y + p.z * p.z;
}

// Verify AOJ 0115
inline double abs(const Point3d &p) { return sqrt(norm(p)); }

inline double toRad(double theta) { return theta * M_PI / 180.0; }

bool on_line3d(Line3d line, Point3d p) {
  return equals(abs(cross(p - line.p[0], line.p[1] - line.p[0])), 0);
}

bool on_segment3d(Segment3d seg, Point3d p) {
  if (!on_line3d(seg, p))
    return false;
  double dist[3] = {abs(seg.p[1] - seg.p[0]), abs(p - seg.p[0]),
                    abs(p - seg.p[1])};
  return on_line3d(seg, p) && equals(dist[0], dist[1] + dist[2]);
}

// Verify AOJ 0115
bool point_on_the_triangle3d(Point3d tri1, Point3d tri2, Point3d tri3,
                             Point3d p) {

  // 線分上にpがあった場合、三角形内とみなす
  // if( on_segment3d(Segment3d(tri1,tri2),p) ) return true;
  // if( on_segment3d(Segment3d(tri2,tri3),p) ) return true;
  // if( on_segment3d(Segment3d(tri3,tri1),p) ) return true;

  Vector3d v1 = tri2 - tri1;
  Vector3d v2 = tri3 - tri2;
  Vector3d v3 = tri1 - tri3;

  Vector3d cp[3] = {cross(v1, p - tri1), cross(v2, p - tri2),
                    cross(v3, p - tri3)};
  double d1 = dot(cp[0], cp[1]);
  double d2 = dot(cp[0], cp[2]);

  // 線分上にpがあった場合,三角形内とはみなさない
  // if( ( !equals(d1,0.0) && d1 > 0 ) && ( !equals(d2,0.0) && d2 > 0 ) ) return
  // true;

  // 線分上にpがあった場合,三角形内とみなす
  if ((equals(d1, 0.0) || d1 > 0) && (equals(d2, 0.0) || d2 > 0))
    return true;
  return false;
}

inline Point3d rotateX(Point3d p, double rad) {
  return Point3d(p.x, p.y * cos(rad) - p.z * sin(rad),
                 p.y * sin(rad) + p.z * cos(rad));
}

inline Point3d rorateY(Point3d p, double rad) {
  return Point3d(p.x * cos(rad) + p.z * sin(rad), p.y,
                 -p.x * sin(rad) + p.z * cos(rad));
}

inline Point3d rorateZ(Point3d p, double rad) {
  return Point3d(p.x * cos(rad) - p.y * sin(rad),
                 p.x * sin(rad) + p.y * cos(rad), p.z);
}

inline Point3d rotateEuler(Point3d p, double alpha, double beta, double gamma) {
  return Point3d(
      (cos(alpha) * cos(beta) * cos(gamma) - sin(alpha) * sin(gamma)) * p.x +
          (-cos(alpha) * cos(beta) * sin(gamma) - sin(alpha) * cos(gamma)) *
              p.y +
          (cos(alpha) * sin(beta)) * p.z,
      (sin(alpha) * cos(beta) * cos(gamma) + cos(alpha) * sin(gamma)) * p.x +
          (-sin(alpha) * cos(beta) * sin(gamma) + cos(alpha) * cos(gamma)) *
              p.y +
          (sin(alpha) * sin(beta)) * p.z,
      (-sin(beta) * cos(gamma)) * p.x + (sin(beta) * sin(gamma)) * p.y +
          (cos(beta)) * p.z);
}

inline Point3d rotateRollPitchYaw(Point3d p, double roll, double pitch,
                                  double yaw) {
  return Point3d(
      (cos(roll) * cos(pitch)) * p.x +
          (cos(roll) * sin(pitch) * sin(yaw) - sin(roll) * cos(yaw)) * p.y +
          (cos(roll) * sin(pitch) * cos(yaw) + sin(roll) * sin(yaw)) * p.z,
      (sin(roll) * cos(pitch)) * p.x +
          (sin(roll) * sin(pitch) * sin(yaw) + cos(roll) * cos(yaw)) * p.y +
          (sin(roll) * sin(pitch) * cos(yaw) - cos(roll) * sin(yaw)) * p.z,
      -sin(pitch) * p.x + cos(pitch) * sin(yaw) * p.y +
          cos(pitch) * cos(yaw) * p.z);
}

class Plane3d {
public:
  Point3d normal_vector; // 法線ベクトル
  double d; // 平面方程式 normal_vector.x * x + normal_vector.y * y +
            // normal_vector.z * z + d = 0

  Plane3d(Point3d normal_vector = Point3d(), double d = 0)
      : normal_vector(normal_vector), d(d) {}
  Plane3d(Vector3d a, Vector3d b, Vector3d c) {
    Vector3d v1 = b - a;
    Vector3d v2 = c - a;
    Vector3d tmp = cross(v1, v2);
    normal_vector = tmp / abs(tmp);
    set_d(a);
  }

  // Verify AOJ 0115
  // 法線ベクトルnormal_vectorと平面上の１点からdを計算する
  void set_d(Point3d p) { d = dot(normal_vector, p); }

  // 平面と点pの距離を求める
  double distanceP(Point3d p) {
    Point3d a = normal_vector * d; // 平面上の適当な点をつくる
    return abs(dot(p - a, normal_vector));
  }

  // 平面上でもっとも点pと近い点を求める
  Point3d nearest_point(Point3d p) {
    Point3d a = normal_vector * d;
    return p - (normal_vector * dot(p - a, normal_vector));
  }

  // Verify AOJ 0115
  // 平面と線分が交差するか
  bool intersectS(Segment3d seg) {
    Point3d a = normal_vector * d;
    double res1 = dot(a - seg.p[0], normal_vector);
    double res2 = dot(a - seg.p[1], normal_vector);
    if (res1 > res2)
      swap(res1, res2);
    // cout << res1 << " < " << res2 << endl;
    if ((equals(res1, 0.0) || res1 < 0) && (equals(res2, 0.0) || res2 > 0))
      return true;
    return false;
  }

  // Verify AOJ 0115
  // 平面と線分の交点を求める
  Point3d crosspointS(Segment3d seg) {
    Point3d a = normal_vector * d;
    double dot_p0a = fabs(dot(seg.p[0] - a, normal_vector));
    double dot_p1a = fabs(dot(seg.p[1] - a, normal_vector));
    if (equals(dot_p0a + dot_p1a, 0))
      return seg.p[0];
    return seg.p[0] + (seg.p[1] - seg.p[0]) * (dot_p0a / (dot_p0a + dot_p1a));
  }
};

double distanceLP(Line3d line, Point3d p) {
  return abs(cross(line.p[1] - line.p[0], p - line.p[0])) /
         abs(line.p[1] - line.p[0]);
}

Point3d project(Segment3d seg, Point3d p) {
  Vector3d base = seg.p[1] - seg.p[0];
  double t = dot(p - seg.p[0], base) / norm(base);
  return seg.p[0] + base * t;
}

Point3d reflect(Segment3d seg, Point3d p) {
  return p + (project(seg, p) - p) * 2.0;
}

// Verify AOJ 2081
bool isParallel(Line3d l1, Line3d l2) {
  Vector3d A = l1.p[0], B = l1.p[1], C = l2.p[0], D = l2.p[1];
  Vector3d AB = B - A, CD = D - C;
  Vector3d n1 = AB / abs(AB), n2 = CD / abs(CD);
  double tmp = dot(n1, n2);
  tmp = 1 - tmp * tmp;
  return equals(tmp, 0.0);
}

// Verify AOJ 2081
// l1,l2が平行な時には使用できないので注意
Segment3d nearest_segmentLL(Line3d l1, Line3d l2) {
  assert(!isParallel(l1, l2)); // 平行な場合は使用不可
  // l1.p[0] = A, l1.p[1] = B, l2.p[0] = C, l2.p[1] = D
  Vector3d AB = l1.p[1] - l1.p[0];
  Vector3d CD = l2.p[1] - l2.p[0];
  Vector3d AC = l2.p[0] - l1.p[0];
  Vector3d n1 = AB / abs(AB), n2 = CD / abs(CD);
  double d1 =
      (dot(n1, AC) - dot(n1, n2) * dot(n2, AC)) / (1.0 - pow(dot(n1, n2), 2));
  double d2 =
      (dot(n1, n2) * dot(n1, AC) - dot(n2, AC)) / (1.0 - pow(dot(n1, n2), 2));
  return Segment3d(l1.p[0] + n1 * d1, l2.p[0] + n2 * d2);
}

// Verify AOJ 2081
bool intersectLL(Line3d l1, Line3d l2) {
  Vector3d A = l1.p[0], B = l1.p[1], C = l2.p[0], D = l2.p[1];

  // そもそもl1,l2が直線じゃない
  if (equals(abs(B - A), 0.0) || equals(abs(D - C), 0.0)) {
    /*
      この場合は注意
      そもそも与えられた線分が線分になっていないので、交差するかどうかは判定できない
    */
    return false;
  }

  Vector3d AB = B - A, CD = D - C;
  Vector3d n1 = AB / abs(AB), n2 = CD / abs(CD);
  double tmp = dot(n1, n2);
  tmp = 1 - tmp * tmp;

  if (equals(tmp, 0.0))
    return 0; // 直線が平行

  Segment3d ns = nearest_segmentLL(l1, l2);
  if (ns.p[0] == ns.p[1])
    return true;
  return false;
}

bool intersectSS(Segment3d seg1, Segment3d seg2) {
  if (isParallel(seg1, seg2))
    return false;
  Segment3d seg = nearest_segmentLL(seg1, seg2);
  if (!(seg.p[0] == seg.p[1]))
    return false;
  Point3d cp = seg.p[1];
  return on_segment3d(seg1, cp) && on_segment3d(seg2, cp);
}

Line3d line[110];
Point3d source_p, sink_p;
int source, sink;
double fw[110][110];

int main() {
  int N;
  while (cin >> N, N) {
    cin >> source_p.x >> source_p.y >> source_p.z >> sink_p.x >> sink_p.y >>
        sink_p.z;

    source = sink = -1;
    rep(i, N) {
      rep(j, 2) cin >> line[i].p[j].x >> line[i].p[j].y >> line[i].p[j].z;
      if (on_line3d(line[i], source_p)) {
        assert(source == -1);
        source = i;
      }
      if (on_line3d(line[i], sink_p)) {
        assert(sink == -1);
        sink = i;
      }
    }
    assert(source != -1 && sink != -1);

    rep(i, N) rep(j, N) fw[i][j] = IINF;
    rep(i, N) REP(j, i, N) {
      if (i == j)
        fw[i][j] = 0;
      else if (isParallel(line[i], line[j])) {
        double area = abs(
            cross(line[i].p[1] - line[i].p[0], line[j].p[1] - line[i].p[0]));
        fw[i][j] = fw[j][i] = area / abs(line[i].p[1] - line[i].p[0]);
      } else if (!intersectLL(line[i], line[j])) {
        Segment3d seg = nearest_segmentLL(line[i], line[j]);
        fw[i][j] = fw[j][i] = abs(seg.p[1] - seg.p[0]);
      }
    }

    rep(k, N) rep(i, N)
        rep(j, N) if (!equals(fw[i][k], IINF) && !equals(fw[k][j], IINF))
            fw[i][j] = min(fw[i][j], fw[i][k] + fw[k][j]);
    printf("%.4f\n", fw[source][sink]);
  }
  return 0;
}