There are `n` gas stations along a circular route. You are given two integer arrays `gas` and `cost` where `gas[i]` is the amount of gas at the `i`th station and `cost[i]` is the cost to travel from the `i`th station to its next station. You begin the journey with an empty tank.
Return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return `-1`. If there exists a solution, it is guaranteed to be **unique**.
**Example 1:**
```
Input: gas = [1,2,3,4,5], cost = [3,4,5,1,2]
Output: 3
```
**Example 2:**
```
Input: gas = [2,3,4], cost = [3,4,3]
Output: -1
```