A cylindrical paper tube is surrounded by a rectangular hard paper with a length of 5 cm and a width of 4 cm. The width of the seam is 1 cm. What is the maximum side area of the paper tube in square centimeters?
用一张长5厘米、宽4厘米的长方形硬纸围成一个圆柱形纸筒，接缝处宽1厘米，纸筒的侧面积最大是多少平方厘米．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<result>16</result>