Idea: To find the minimum positive integer speed that allows you to reach the office in the given time, you can use binary search on the speed. Initialize the left boundary to 1 (minimum speed) and right boundary to a large number (e.g., 10^7, as specified in the problem statement), then narrow down the possible speed by simulating the commute.

Steps:

• Perform binary search on speed from 1 to 10^7.
• For each candidate speed, iterate over all trains, compute the total time taken, rounding up the time taken for all but the last train.
• If total time is less than or equal to given hour, try with a smaller speed. Otherwise, try with a larger speed.

If you narrow down to a result, it is the minimum speed required. If search is exhausted without finding a valid speed, return -1.