input: Write a function to extract the title from the given HTML string.
  assert extract_title('<!DOCTYPE html><html><head><title>Basic Website</title></head><body></body></html>') == 'Basic Website'
  assert extract_title('<html><head><title>Example Title</title></head><body></body></html>') == 'Example Title'
  assert extract_title('<html><head><title>Another Test</title></head><body></body></html>') == 'Another Test'
output: def extract_title(html):
      start_index = html.find('<title>') + len('<title>')
      end_index = html.find('</title>')
      return html[start_index:end_index]