Multiplying a two-digit number is called an "operation". If the product is still a two-digit number, repeat the above operation until a one-digit number is obtained, for example: 29→2*9=18→1*8 =8(Stop>A total of two operations have been performed, a two-digit number has undergone three operations as above, and finally a one-digit number is obtained. What is the minimum number of this two-digit number.
将一个两位数的数字相乘，称为一次"操作"，如果积仍是一个两位数，重复以上操作，直到得到一个一位数，例如：29→2*9=18→1*8=8(停止＞共经历两次操作，一个两位数经过3次如上操作，最终得到一位数，这个两位数最小是多少．
<result>39</result>