"We need to fly home as cheaply as possible so that more money is left for gifts. Aunt Lidia asked for different kinds of cheeses, and Vasia wanted a new toy car. I’ve been looking at the schedule for quite a while and I’m starting to think that some planes are flying in vain".

As the input you get the flight schedule as list, each element of which is the price of a direct flight between 2 cities (list of 3 elements - 2 city names as a string, and a flight price).

Planes fly in both directions and the price in both directions is the same. There is a possibility that there are no direct flights between cities.

Find the price of the cheapest flight between cities that are given as the 2nd and 3rd arguments. In case the flight can not be performed, return 0.

Input: 3 arguments: the flight schedule as a list of lists, city of departure and destination city as strings.

Output: The best price as integer.

assert (
    cheapest_flight([["A", "C", 100], ["A", "B", 20], ["B", "C", 50]], "A", "C") == 70
)
assert (
    cheapest_flight([["A", "C", 100], ["A", "B", 20], ["B", "C", 50]], "C", "A") == 70
)
assert (
    cheapest_flight(
        [
            ["A", "C", 40],
            ["A", "B", 20],
            ["A", "D", 20],
            ["B", "C", 50],
            ["D", "C", 70],
        ],
        "D",
        "C",
    )
    == 60
)
assert (
    cheapest_flight([["A", "C", 100], ["A", "B", 20], ["D", "F", 900]], "A", "F") == 0
)

How it can be used: In everyday life to find the optimal combination.

Preconditions:

• price is always integer;
• the flight schedule contains at least one element;
• both cities are in the schedule.