A fraction can be reduced to (1/2) if its denominator is added with 1, and it can be reduced to (2/3) if its denominator is subtracted from 1. what is this number
一个分数，若它的分母加上1得到的分数可约简为(1/2)，若分母减去1得到的分数可约简为(2/3)．这个数是多少
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<result>4/7 = around 0.571429</result>