40. At the opening ceremony of the Green Green Grassland Sports Meet, there are 2020 balloons arranged in a row from left to right, numbered sequentially as 1, 2, 3, ..., 2020. The first time, the balloons with numbers that are perfect squares are released. Then, the remaining balloons are renumbered from 1 from left to right. The second time, the balloons with numbers that are perfect squares are released again, and the remaining balloons are renumbered... So, the initial number of the last balloon to be released is . $\qquad$
$1981$