You are given an array `routes` representing bus 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 -> 1 -> ...` forever.

You will start at the bus stop `source` (You are not on any bus initially), and you want to go 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.

Example 1:

    Input: routes = [[1,2,7],[3,6,7]], source = 1, target = 6
    Output: 2
    Explanation: The best strategy is take the first bus to the bus stop 7, then take the second bus to the bus stop 6.

Example 2:

    Input: routes = [[7,12],[4,5,15],[6],[15,19],[9,12,13]], source = 15, target = 12
    Output: -1

Constraints:

- `1 <= routes.length <= 500`.
- `1 <= routes[i].length <= 105`
- All the values of `routes[i]` are unique.
- `sum(routes[i].length) <= 105`
- `0 <= routes[i][j] < 106`
- `0 <= source, target < 106`