Bus Routes

You are given an array routes where routes[i] is a bus route that the ith bus repeats forever. For example, if routes[0] = [1, 5, 7], this means that the 0th bus travels in the sequence 1 -> 5 -> 7 -> 1 -> 5 -> 7 -> ... forever. You will start at the bus stop source and want to travel to the bus stop target. You can travel between bus stops by buses only. Return the least number of buses you must take to travel from source to target. Return -1 if it is not possible.

Take the first bus from stop 1 to stop 7, then transfer to the second bus to reach stop 6.