input: def num_unique_paths(m: int, n: int) -> int:
      """ Find the number of unique paths from a given source to a destination in a m x n grid.
      The only moves allowed are right and down.
      >>> 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)