old_contents: from flask import Flask, jsonify, render_template, request
  app = Flask(__name__)
  
  
  @app.route('/_add_numbers')
  def add_numbers():
      ''' Because numbers must be added server side '''
      a = request.args.get('a', 0, type=int)
      b = request.args.get('b', 0, type=int)
      return jsonify(result=a + b)
  
  
  @app.route('/')
  def index():
      return render_template('index.html')
  
new_contents: from flask import Flask, jsonify, render_template, request
  app = Flask(__name__)
  
  
  @app.route('/_add_numbers')
  def add_numbers():
      ''' Because numbers must be added server side '''
      a = request.args.get('a', 0, type=int)
      b = request.args.get('b', 0, type=int)
      return jsonify(result=a + b)
  
  
  @app.route('/')
  def index():
      return render_template('index.html')
  
  if __name__ == '__main__':
      app.run('0.0.0.0',port=4000)
  
subject: Add app run on main