input: from typing import List
  
  def num_unique_paths(m: int, n: int) -> int:
      """
      Calculate the number of unique paths from the top-left corner to the bottom-right corner of an m x n grid.
      The only moves allowed are right and down.
  
      >>> num_unique_paths(3, 2)
      3
      >>> num_unique_paths(7, 3)
      28
      >>> num_unique_paths(1, 1)
      1
      """
output:     if m == 1 or n == 1:
          return 1
      
      dp = [[0] * n for _ in range(m)]
      
      for i in range(m):
          dp[i][0] = 1
      
      for j in range(n):
          dp[0][j] = 1
      
      for i in range(1, m):
          for j in range(1, n):
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
      
      return dp[m-1][n-1]