input: 
  def concatenate_strings(strings, separator):
      """Concatenates a list of strings with a specified separator.
      >>> concatenate_strings(['Sample', 'Text', 'Hello'], ' ')
      'Sample Text Hello'
      >>> concatenate_strings(['Sample', 'Text', 'Hello'], ',')
      'Sample,Text,Hello'
      """
  
output:     result = ""
      for i, s in enumerate(strings):
          result += s
          if i < len(strings) - 1:
              result += separator
      return result