Two Trains

Two trains on one track, racing toward each other — when do they meet? (Relax, they're toy trains.) 🚂🚃

Train A starts at position a moving right sa per second; train B starts at b (further right) moving left sb per second. Print the first whole second when A's position is at or past B's.

Example:

Input: a = 0, sa = 3, b = 12, sb = 1 Output: 3

The gap between them (12) shrinks by 3 + 1 = 4 each second, closing after 12 ÷ 4 = 3 seconds.