input: from typing import List
  
  def num_unique_paths(m: int, n: int) -> int:
      """Returns the number of unique paths from the top-left corner to the bottom-right corner 
      in a grid of size m x n.
      
      >>> num_unique_paths(3, 2)
      3
      >>> num_unique_paths(7, 3)
      28
      """
output:     if m == 1 or n == 1:
          return 1
      return num_unique_paths(m-1, n) + num_unique_paths(m, n-1)