input: def concatenate_strings(strings, separator):
      """
      Takes a list of strings and returns the concatenation of all the elements with a given separator.
      
      >>> concatenate_strings(["Sample","Text","Hello"], "-")
      'Sample-Text-Hello'
      >>> concatenate_strings(["1", "2", "3"], ",")
      '1,2,3'
      """
output:     return separator.join(strings)