My IF statement does not run how I would expect with my OR operator, it only runs my Else if both are strings?

I set up this challenge a little differently. I am using prompts and not console.log. I do not know what I am doing wrong in my code.

This is what happens:

• If I answer the first prompt with a string and the second with a number, my result gives no error alert and my final alert is NaN

• If I answer the first prompt with a string and the second with a string,, my result gives the correct error alert, but my final alert is NaN

• If I answer both prompts with a number, it runs correctly, it results in my error alert not running and my random number shows correctly.

• What I want to happen: If the first prompt answer is a string , it should show the error alert, before showing the second prompt. If the second prompt is a string then it should show the error alert again.

function randomCalculation(upper, lower) {
  if ( !isNaN(upper) || !isNaN(lower) ) {

    return Math.floor(Math.random() * (upper - lower + 1)) + lower;

  } else {
    alert('Error! That is not a number');

  }
}


// Two Prompts are run  and each answer is stored in a variable below

var answerOne = parseInt(prompt('Please Input Your Upper Number'));
var answerTwo = parseInt(prompt('Please Input Your Lower Number'));



// The above variables are run as arguments of the function, and
// The result is assigned to the variable below

var runMyProgram = randomCalculation(answerOne,answerTwo);


// We run the above variable
alert( runMyProgram );