Stop javascript treating a number as text

gistfile1.js

// This does not work and gives the wrong figure due to concatination 
var shiftPrem = $('#Contract_Reward_ShiftPremium').val();
var basePay = $('#Contract_Reward_BasePay').val();
var total = basePay + (basePay * (shiftPrem/100));

//This works by forcing javascript to treat the variables as numbers
shiftPrem = $('#Contract_Reward_ShiftPremium').val();
basePay = $('#Contract_Reward_BasePay').val();
total = +basePay + +(basePay * (shiftPrem/100));