A is the continuous product of four numbers 870, 935, 972 and a (a is a natural number). If the last four numbers of A are all zero, what is the minimum value of a?
A是四个数870、935、972、a(a是自然数)的连乘积，如果要求A的最后四个数字都是零，a最小应是多少．
<gadget id="calculator">5 * 5 * 2</gadget>
<output>50</output>

<result>50</result>