There are 21 logs piled up in the shape of a pagoda, one on the top layer, and one log more than the previous layer on each lower layer. Think about how many logs there are on the bottom layer.
有21根圆木，堆成宝塔形，最上面一层放一根，下面每一层都比上一层多1根，想想看，最下面一层有多少根．
<result>6</result>