There are 7 line segments of different lengths, but all are whole centimeters. The shortest one is 1 cm long, and any three of them cannot be used as sides to form a triangle. What is the minimum length of the longest of these seven line segments?
有7条长度不同，但都是整厘米数的线段．其中最短的一条长1厘米，并且任何三条都不能作为边构成三角形．这七条线段中最长的线段至少长多少厘米．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

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

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

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

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

<result>21</result>