How to get current frame in expressions.

I am trying to make the opacity of a video 0 on every even frame, and 100 on every odd. I have added an expression on the opacity of the video that looks like this:

d = thisComp.frameDuration;
if (d%2==0){
opacity = 0;
}else{
opacity = 100;
}

but this does not change the opacity of the video at any time.

how would I get the current frame number, then test every frame to see if it is even or odd and adjust the opacity accordingly.

you could do this (where 25 is fps):

(time * 25) % 2 == 0