store stipulates that 3 empty soda bottles are exchanged for a bottle of soda, and a person needs to buy at least as many bottles of soda in this store to drink 21 bottles of soda.
某商店规定，3个空汽水瓶换一瓶汽水，某人在这个商店至少需购买多少瓶汽水就可以喝到21瓶汽水．
<result>14</result>