input: Write a function to calculate the number of unique paths from the top-left corner to the bottom-right corner of a grid with dimensions `m` x `n`, only moving right or down. The function should return the number of unique paths.
  
  def num_unique_paths(m, n):
      dp = [[0 for x in range(m)] for y in range(n)] 
    
      for i in range(n): 
          dp[i][0] = 1
    
      for j in range(m): 
          dp[0][j] = 1
    
      for i in range(1, n): 
          for j in range(1, m): 
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
    
      return dp[n-1][m-1]
  
  assert num_unique_paths(2, 3)==3
  assert num_unique_paths(3, 7)==28
  assert num_unique_paths(1, 1)==1
  assert num_unique_paths(4, 4)==20
  assert num_unique_paths(0, 0)==0
output: def num_unique_paths(m, n):
      if m == 0 or n == 0:
          return 0
      dp = [[0 for x in range(m)] for y in range(n)] 
    
      for i in range(n): 
          dp[i][0] = 1
    
      for j in range(m): 
          dp[0][j] = 1
    
      for i in range(1, n): 
          for j in range(1, m): 
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
    
      return dp[n-1][m-1]