'use strict';

function calculator() {

	this.read = function(){
		this.a = prompt("a?", 0);
		this.b = prompt("b?", 0);
	};

	this.sum - function() {
		return this.a + this.b;
	};

	this.mul = function() {
		return this.a * this.b;
	};
}

let calculator = new Calculator();
calculator.read();

alert( "Sum" + calculator.sum() );
alert( "Mul" + calculator.mul() );