Newbie Javascript: Remaining calories

I am very new and practicing functions right now

I made this to create a simple remaining calories calculator, but I wanted to ask:

When I have originally created a value that already calculates the difference between, lets say, caloriesMen and caloriesToday and named it caloriesLeft (so I literally wrote ' let caloriesLeft = caloriesMen - caloriesToday ') but it came up with NaN which is somewhat understandable.

But when I did that in the alert() it understood what I wanted (e.g. 'You have ' + (caloriesMen - caloriesToday) + ' calories left to consume today!')

1. Could you tell me why is that?

2. How would you made this code (better/cleaner)?

My code and vars:

let gender;
let caloriesMen = 2500;
let caloriesWomen = 2000;
let caloriesToday;

function calCalc() {

gender = prompt('Are you man or woman?') 
    if (gender == 'man') {
        caloriesToday = prompt('How many calories have you already consumed today?');
        alert('You have ' + (caloriesMen - caloriesToday) + ' calories left to consume today!');
    } else if (gender == 'woman') {
        caloriesToday = prompt('How many calories have you already consumed today?');
        alert('You have ' + (caloriesWomen - caloriesToday) + ' calories left to consume today!');
    }
    
}

calCalc();

Thank you!