It is known that 1999*△+4*□=9991, where △ and □ are natural numbers, then □=how many.
已知1999*△+4*□=9991，其中△，□是自然数，那么□=多少．
<gadget id="calculator">1_999 * 1</gadget>
<output>1_999</output>

<gadget id="calculator">9_991 - 1_999</gadget>
<output>7_992</output>

<gadget id="calculator">7_992 / 4</gadget>
<output>1_998</output>

<result>1_998</result>