input: Write a function to create a web API using Flask that provides the current date and time. The function should return the current date and time in the format "YYYY-MM-DD HH:MM:SS".
  
  assert get_datetime().startswith("20")
  assert len(get_datetime()) == 19
output: from flask import Flask, jsonify
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/datetime')
  def get_datetime():
      now = datetime.now()
      return now.strftime("%Y-%m-%d %H:%M:%S")