1999/1998*(1998/1999).
1999/1998*(1998/1999)．
<gadget id="calculator">1_999 / 1_998</gadget>
<output>1_999/1_998 = around 1.000501</output>

<gadget id="calculator">1_998 / 1_999</gadget>
<output>1_998/1_999 = around 0.9995</output>

<gadget id="calculator">(1_999/1_998) * (1_998/1_999)</gadget>
<output>1</output>

<result>1</result>