input: Write a Python function to compute the number of days an employee is available for work without any scheduled meetings. The function should take a positive integer `days` representing the total workdays (starting from day 1) and a 2D list `meetings` where each sublist contains two integers `[start_i, end_i]` indicating the start and end days of the i-th meeting (inclusive). Your task is to return the number of days the employee is free of meetings.
  
  Take note:
  Meetings can overlap.
  
  Examples:
  >>> days = 10, meetings = [[5,7],[1,3],[9,10]]
  2
  >>> days = 5, meetings = [[2,4],[1,3]]
  1
  
  Constraints:
  1 <= days <= 10^9
  1 <= len(meetings) <= 10^5
  Each list in meetings contains exactly 2 integers
  1 <= meetings[i][0] <= meetings[i][1] <= days
output: def free_days(days, meetings):
      busy_days = set()
      for start, end in meetings:
          for day in range(start, end + 1):
              busy_days.add(day)
      return days - len(busy_days)