1038. Where to Start the Circuit

n fuel stations stand in a circle. Station i gives you gas[i] units, and driving from station i to the next one costs cost[i] units. You start with an empty tank and may begin anywhere.

Print the 0-based index of a station you can start from and complete the full circle without the tank ever going negative, or -1 if no such station exists. When more than one works, print the smallest such index.

Trying every start costs O(n^2). Two observations reduce it to one pass.

First, a circuit is possible at all only when the total gas is at least the total cost. That single sum decides between an answer and -1, before any start is considered.

Second, suppose you begin at s and the tank first goes negative on arriving at station j. Then no station between s and j can be the answer either. Each such station was reached with a tank that was zero or more, so starting there gives you no more fuel than you already had, and you would run dry no later. So the next candidate is j + 1, and the scan never goes back.

Together: one pass tracking the running tank, resetting the candidate start whenever it falls below zero, plus the total to decide whether an answer exists.

Constraints - `1 ≤ n ≤ 200000` - `0 ≤ gas[i] ≤ 100000` - `0 ≤ cost[i] ≤ 100000`