input: 
  from flask import Flask
  import datetime
  
  def get_current_datetime() -> str:
      """ Create a web API using Flask that returns the current date and time in the format 'YYYY-MM-DD HH:MM:SS'.
      >>> get_current_datetime()
      '2023-10-01 14:30:45'
      """
      return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
output:     
  app = Flask(__name__)
  
  @app.route('/current_datetime')
  def current_datetime():
      return get_current_datetime()
  
  if __name__ == '__main__':
      app.run()
  