how to count only if light sensor event going from 100 to 50 and don't count if it from 0 to 50

Question

How to count only if light sensor event going from 100 to 50 and don't count if it from 0 to 50 ?

I'm using light sensor in my app( onsensorchange event ) I want to count only if light fade from 100 to 50% then add +1 to my counter

button1.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        sen = //get value of event.values[0] 
    }
});

public void onSensorChanged(SensorEvent event) {
    if (event.sensor.getType() == Sensor.TYPE_LIGHT) {
        if (sen => half of event.values[0]) {
            // add +1 to my counter
        }
    }
}

the problem is they count when event.values[0] goes from 100 to 50 and from 0 to 50