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