Leetcode 1870. Minimum Speed to Arrive on Time

Find the minimum positive integer speed such that, when intermediate trains can only depart at integer hours (so each of the first n-1 rides contributes ceil(dist[i]/speed) hours and the last ride contributes dist[n-1]/speed hours), the total commute time ≤ hour; return -1 if no such speed exists. This is a monotonic feasibility check over speed (suitable for binary search).