If an represents the last digit of 1997n, then a1+a2+a3+...+a2007=how much.
若an表示1997n的末位数字，则a1+a2+a3+…+a2007=多少．
<gadget id="calculator">7 + 9 + 3 + 1</gadget>
<output>20</output>

<gadget id="calculator">20 * 501</gadget>
<output>10_020</output>

<gadget id="calculator">7 + 9 + 3</gadget>
<output>19</output>

<gadget id="calculator">10_020 + 19</gadget>
<output>10_039</output>

<result>10_039</result>