The greatest 4 digit number which is a perfect square, is = ?

$$\eqalign{ & {\text{Greatest 4 digit number}} \cr & {\text{ = 9999 }} \cr & \,\,\,\,\,\,\,\,{\text{9| }}\,{\text{9999 | 99}} \cr & \,\,\,\,\,\,\,\,\,\,\,|\,\,\,81 \cr & \overline {189\,\,|\,\,\,1899} \cr & \,\,\,\,\,\,\,\,\,\,\,|\,\,\,1701 \cr & \,\,\,\,\,\,\,\,\,\,\,\overline {|\,\,\,\,\,198} \cr} $$
Greatest four digit number is a perfect square
= 999 - 198
= 9801