probleme with if condition in java function

I use a radio buton 'plata' with two values '1' and '2' in my form and another input hidden 'plata2' take one value depend to 'plata'.

So i use this code inside in an existant function total():

test = document.getElementById('plata').value;
if (test == 1) {document.getElementById('plata2').value=pretfinal;}
 else {document.getElementById('plata2').value=avansfinal;}

but 'plata2' take just first value, 'else' never works? can help with this. THX