input: Write a python function to create a Flask RESTful API that adds two integers and returns their sum.
  assert sum_api(5, 10) == {'sum': 15}
  assert sum_api(3, 7) == {'sum': 10}
  assert sum_api(-1, 1) == {'sum': 0}
output: from flask import Flask, request, jsonify
  
  app = Flask(__name__)
  
  @app.route('/add', methods=['POST'])
  def sum_api():
      data = request.get_json()
      num1 = data['num1']
      num2 = data['num2']
      result = num1 + num2
      return jsonify({'sum': result})
  
  if __name__ == '__main__':
      app.run(debug=True)