Minimum Transfer Route

Given information about the subway lines in a city, write a program that finds the minimum number of transfers needed to travel from a starting station to a destination station. You do not need to output an actual route; output only the number of transfers.

The first line contains the number of stations N (1 <= N <= 100,000) and the number of lines L (1 <= L <= 100,000).

Each of the next L lines lists, in order, the stations passed by one subway line. Each such line ends with -1.

The last line contains the starting station number and the destination station number. Station numbers are integers from 1 to N. The sum of the lengths of all subway lines does not exceed 1,000,000.

Print the minimum number of transfers. If the destination cannot be reached, print -1.