The NaN is an easy fix
Nice and simple application, but if you want to fix the NaN problem (and make it more idiot-proof) just use a simple function when taking the number from the textboxes, in one of two ways:
var exp = getVal(exp_txt.text); //exp_txt is the instance NAME of the textbox
//continue to grab each variable using the getVal() function, as defined below...
function getVal(str){
str = parseInt(str, 10);
if (isNaN(str)) return 0;
else return str;
}
//========
//if instead of giving the textboxes NAMES you gave them VARIABLES, then you can modify before it is evaluated, like this:
exp = getVal(exp); //this will, (if exp is the variable in the input box), correct the value if the user types in something that isnt numerical, or leaves it blank. You will still need to copy and paste the function above.
Hope it helps! =D