Dad bought a cylindrical birthday cake for his daughter Yuanyuan. Yuanyuan wants to cut the cake into several pieces (not less than 10 pieces) that are not necessarily equal in size and distribute them to 10 children. If the cake is divided vertically A piece of cake, at least how many knives need to be cut.
爸爸给女儿圆圆买了一个圆柱形的生日蛋糕，圆圆想把蛋糕切成大小不一定相等的若干块(不少于10块)，分给10个小朋友，若沿竖直方向切分这块蛋糕，至少需要切多少刀．
<result>4</result>