Field with Script - doesn't execute unless I edit the same or any other field

Hello

I have a $ field that I check to see if it's >= to another.  If it is - I want the Next Field to Not Display

It works but I unfortunately have to click into any field again and step off for the script to execute. 

var v1 = Number(this.getField("Step2_a").value);
var v2 = Number(this.getField("Step2_b").value);

if (v2 >= v1) {
this.getField("Step2_c").display = display.hidden;
}

else {
this.getField("Step2_c").display = display.visible;
}