There are 7 steps in a staircase, and one or two steps can be taken at a time when going upstairs. How many different ways are there to go from the ground to the top floor?
一个楼梯有7阶，上楼时每次可以跨一阶或两阶．从地面到最上层共有多少种不同的走法．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">2 + 3</gadget>
<output>5</output>

<gadget id="calculator">3 + 5</gadget>
<output>8</output>

<gadget id="calculator">5 + 8</gadget>
<output>13</output>

<gadget id="calculator">8 + 13</gadget>
<output>21</output>

<result>21</result>