Two different methods which should produce the same outcome. What is wrong in this code?

Hello community,

Would you kindly support me with the following problem? The code that I wrote is these:

/**

• Returns a random number between two numbers. *
• @param {number} lower - The lowest number value.
• @param {number} upper - The highest number value.
• @return {number} The random number value. */

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

// 1st method: Calls the function by writing the two arguments directly in the code ==> Outcome: plausible! console.log(getRandomNumber(20,100));

// 2nd method: Asks the user for input and assign it to the function ==> Outcome: completely wrong.

const lower = prompt(Enter the lower number.);

const upper = prompt(Enter the upper number.);

console.log(getRandomNumber(lower,upper));

Why does the 2nd method is returning totally weird values? If I insert for example 20 and 100 via "prompt", I get a number between like 410 or 1020.

Could somebody explain to me why this is happening?

Thank you very much in advance.