input: Write a function to reverse the characters of a given string argument and return the reversed string.
  def reverse_string(input_string):
      reversed_string = ''
      for i in range(len(input_string)-1, -1, -1):
          reversed_string += input_string[i]
      return reversed_string
  
  assert reverse_string("hello") == "olleh"
  assert reverse_string("Python") == "nohtyP"
  assert reverse_string("") == ""
  assert reverse_string("12345") == "54321"
  assert reverse_string("madam") == "madam"
output: def reverse_string(input_string):
      return input_string[::-1]