input: def get_current_datetime_api():
      """ Create a web API with Python and Flask to give the current date and time.
      The API should return the current date and time in the format "YYYY-MM-DD HH:MM:SS".
      Example:
      When the API endpoint '/datetime' is accessed, it should return the current date and time as a string.
      """
      from flask import Flask
      import datetime
  
      app = Flask(__name__)
  
      @app.route('/datetime')
      def get_datetime():
          return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
  
      if __name__ == '__main__':
          app.run()
output:     return app