How to correctly use values from textboxes in html

I wanted to use values from textboxes but failed so i tried with constant values and now I am getting a NAN error. I am showing my result in a label btw.

function myFunction() {
var translength = 2400
var transSpacing = 150
var transEndOverhang = 75
var transStartOverhang = 75

var longLength = 6000
var LongSpacing = 150
var LongStartOverhang = 75
var LongEndOverhang = 75




if ( transSpacing !=0)
 document.getElementById('longAmount').value = ((transLength - transStartOverhang - transEndOverhang) / transSpacing) +1;
   document.getElementById('longAmount').innerHTML =  document.getElementById('longAmount').value
 if ( document.getElementById('longAmount').value > 0 && transStartOverhang + ((document.getElementById('longAmount').value -1) * transSpacing) + transEndOverhang < transLength)
 document.getElementById('longAmount').value = longAmount +1;
  document.getElementById('longAmount').innerHTML = document.getElementById('longAmount').value
}