There are 7 steps in the school teaching stairs, and it is stipulated that one step can only go up one or two steps. How many different ways are there from the bottom to the top?
校教学楼梯有7级，规定一步只能上一级或两级，从最底下上到上面共有多少种不同的上法？
<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>