old_contents: from flask import Flask
  app = Flask(__name__)
  
  @app.route('/')
  def hello_world():
      return 'Hello World!'
  
  if __name__ == '__main__':
      app.run()
new_contents: import os
  from flask import Flask
  
  app = Flask(__name__)
  
  @app.route("/")
  def hello():
      return "Hello world!"
  
  if __name__ == "__main__":
      port = int(os.environ.get("PORT", 5000))
      app.run(host='0.0.0.0', port=port)
subject: Change to use port