Problem - Gas Station

Gas stations on a circular route. Station i has gas[i] fuel and costs cost[i] to reach the next station. Find the starting station to complete the circuit, or return $-1$ if impossible.

Example: gas = [1,2,3,4,5], cost = [3,4,5,1,2]. Start at station $3$ ($0$-indexed). Can you complete the circuit? What is the greedy approach? Constraint: $n \le 10^4$. Simulating every start is $O(n^2)$ time using $O(1)$ space. Greedy gives $O(n)$.