ffmpeg - How to set circle mask using geq filter

I can creat rectangle mask but not creat circle mask. What do I set geq filter options?

rectangle mask setting

ffplay -f lavfi color,geq='st(0,between(X,W/3,2*W/3)*between(Y,H/3,2*H/3));ld(0)*255:128:128'

circle mask image sample