A number sequence has the following rules: when the number n is odd, the next number is (n+1); when the number n is even, the next number is ((n)/2). If the first number in this column is odd and the fourth number is 13, what is the first number in this column?
1个数列有如下规则：当数n是奇数时，下一个数是(n+1)；当数n是偶数时，下一个数是((n)/2)．如果这列数的第1个数是奇数，第4个数是13，则这列数的第一个数是多少．
<gadget id="calculator">13 * 4</gadget>
<output>52</output>

<gadget id="calculator">52 - 1</gadget>
<output>51</output>

<result>51</result>