input: 
  def current_datetime_web_api() -> str:
      """ Create a simple web API using Flask to return the current date and time.
      The API should be accessible via the route '/datetime' and return a string
      formatted as 'YYYY-MM-DD HH:MM:SS'.
  
      Example:
      Accessing '/datetime' should return the current date and time such as:
      "2023-10-05 14:30:00"
      """
  
output:     
      from flask import Flask
      from datetime import datetime
  
      app = Flask(__name__)
  
      @app.route('/datetime')
      def get_current_datetime():
          now = datetime.now()
          return now.strftime('%Y-%m-%d %H:%M:%S')
  
      if __name__ == '__main__':
          app.run()
  