The reliability group at your company runs a long lived service fleet, and every outage lands in a shared incident log stamped with the calendar day it was detected. Each stamp follows the strict YYYY-MM-DD format with leading zeros for two digit months and days, for example 2021-03-09.

During an incident review you are handed two stamps from the log, date1 and date2. Implement solve(date1, date2) to report how many calendar days separate the two recorded days, meaning the count of midnight crossings between them. If both stamps name the same day the answer is 0, and the answer never depends on which incident appears first, because distance is symmetric.