N gas stations are placed along a circular path and each station i has available[i] amount of gas.
There is a bus with an unlimited gas tank and requirement[i] of gas is required to travel from station i to the next station i+1.
The bus starts with an empty tank from one of the gas stations.
Write a method to check whether you can travel around the circular route once else, return the starting gas station’s index (0 to n-1). If no route exists, return -1 .
Assume that a unique solution exists.