Write three natural numbers 100, 200, and 400 on the cardboard, and then write two natural numbers, and then select several (at least two) from these five numbers to do the four arithmetic operations that only add and subtract. In the case, the selected number can only appear once, and after all such operations, k different non-zero natural numbers can be obtained. So what is the largest k?
纸板上写着100、200、400三个自然数，再写上两个自然数，然后从这五个数中选出若干个(至少两个)做只有加、减法的四则运算，在一个四则运算式子中，选出的数只能出现一次，经过所有这样的运算，可以得到k个不同的非零自然数．那么k最大是多少？
<result>64</result>