Improving the random number generator challenge with functions.

Here is my solution for this challenge. I had to copy the method of how to generate a random number between two numbers because I don't understand the math.

Math.random();

generates a random number between 0 and 1 but not including 1. Guil Hernandez said to get it to include up to 1 you add 1.

Math.random() + 1;

to get it to generate a random number between 1 and 6 you multiply by 6 then add 1

Math.random() * 6 + 1;

to get it to generate a number between two numbers it's

Math.random() * (6 -1 + 1) + 1;

where 6 is equal to the upper number and the minus 1 is equal to the lower number and you add 1 then you add back the lower number so this code could be rewritten this way.

Math.random() * (8 - 4 + 1) + 4;

this would generate a random number between 4 and 8. but I don't understand that math.

Anyway once I got that it was easy to figure out the rest. here is my solution.

function getRandomNumber ( lower, upper) {
  var randomNumber = Math.floor(Math.random() * (parseInt(upper) - parseInt(lower) + 1)) + parseInt(lower);
  return randomNumber;
}

document.write( getRandomNumber(0, 6) );
console.log( getRandomNumber(0, 10) );

A lot less code than the first random number game. Just for fun I went ahead and changed it so it would accept user input. Please provide some feedback. thanks

function getRandomNumber ( lower, upper) {
  var randomNumber = Math.floor(Math.random() * (parseInt(upper) - parseInt(lower) + 1)) + parseInt(lower);
  return randomNumber;
}

var lower = prompt('Type a number.');
var upper = prompt('Type a number higher than the number you just typed.');

document.write( getRandomNumber(lower, upper ) );