human
What is a Python program to find the best route which leads to the goal city going through several cities on the road network given the following:
Consider car trips in a country where there is no friction. Cars in this country do not have engines. Once a car started to move at a speed, it keeps moving at the same speed. There are acceleration devices on some points on the road, where a car can increase or decrease its speed by 1. It can also keep its speed there. Your job in this problem is to write a program which determines the route with the shortest time to travel from a starting city to a goal city.

There are several cities in the country, and a road network connecting them. Each city has an acceleration device. As mentioned above, if a car arrives at a city at a speed v , it leaves the city at one of v - 1, v , or v + 1. The first road leaving the starting city must be run at the speed 1. Similarly, the last road arriving at the goal city must be run at the speed 1.

The starting city and the goal city are given. The problem is to find the best route which leads to the goal city going through several cities on the road network. When the car arrives at a city, it cannot immediately go back the road it used to reach the city. No U-turns are allowed. Except this constraint, one can choose any route on the road network. It is allowed to visit the same city or use the same road multiple times. The starting city and the goal city may be visited during the trip.

For each road on the network, its distance and speed limit are given. A car must run a road at a speed less than or equal to its speed limit. The time needed to run a road is the distance divided by the speed. The time needed within cities including that for acceleration or deceleration should be ignored.

Input

The input consists of multiple datasets, each in the following format.

> n  m
>  s  g
>  x 1 y 1 d 1 c 1
>  ...
>  xm ym dm cm
>

Every input item in a dataset is a non-negative integer. Input items in the same line are separated by a space.

The first line gives the size of the road network. n  is the number of cities in the network. You can assume that the number of cities is...
Using python3 can you solve the prior task?
assistant
#Here is Python program to find the best route which leads to the goal city going through several cities on the road network.

import math,string,itertools,fractions,heapq,collections,re,array,bisect,sys,random,time,copy,functools

sys.setrecursionlimit(10**7)
inf = 10**20
eps = 1.0 / 10**10
mod = 10**9+7
dd = [(-1,0),(0,1),(1,0),(0,-1)]
ddn = [(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def LI_(): return [int(x)-1 for x in sys.stdin.readline().split()]
def LF(): return [float(x) for x in sys.stdin.readline().split()]
def LS(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline())
def F(): return float(sys.stdin.readline())
def S(): return input()
def pf(s): return print(s, flush=True)

def main():
    rr = []

    while True:
        n,m = LI()
        if n == 0:
            break

        s,g = LI()
        a = [LI() for _ in range(m)]
        e = collections.defaultdict(list)
        for x,y,d,c in a:
            e[x].append((y,d,c))
            e[y].append((x,d,c))

        def search(s):
            d = collections.defaultdict(lambda: inf)
            d[(s,0,-1)] = 0
            q = []
            heapq.heappush(q, (0, (s, 0, -1)))
            v = collections.defaultdict(bool)
            while len(q):
                k, u = heapq.heappop(q)
                if v[u]:
                    continue
                v[u] = True
                if u[0] == g and u[1] == 1:
                    return '{:0.9f}'.format(k)
                c = u[1]
                b = u[2]

                for uv, ud, uc in e[u[0]]:
                    if uv == b:
                        continue
                    for tc in range(max(c-1,1),min(uc+1,c+2)):
                        nuv = (uv, tc, u[0])
                        if v[nuv]:
                            continue
                        vd = k + ud / tc
                        if d[nuv] > vd:
                            d[nuv] = vd
                            heapq.heappush(q, (vd, nuv))

            return 'unreachable'

        rr.append(search(s))

    return '\n'.join(map(str, rr))


print(main())