GasStation.py

'''
There are N gas stations along a circular route,
where the amount of gas at station i is gas[i].
You have a car with an unlimited gas tank 
and it costs cost[i] of gas to travel from station i to its next station (i+1).
You begin the journey with an empty tank at one of the gas stations.

Return the starting gas station's index if you can travel around the circuit once,
otherwise return -1.

Note:
The solution is guaranteed to be unique.
'''