After adding the same number to the numerator and denominator of the fraction (1985/1987), the resulting fraction is equal to (1989/1990). What is the added number?
分数(1985/1987)的分子、分母同时加上同一个数后，所得的分数等于(1989/1990)，加上的数是多少．
<gadget id="calculator">1_989 / 1_990</gadget>
<output>1_989/1_990 = around 0.999497</output>

<gadget id="calculator">(1_989/1_990) * 1_987</gadget>
<output>3_952_143/1_990 = around 1_986.001508</output>

<gadget id="calculator">(3_952_143/1_990) - 1_985</gadget>
<output>1_993/1_990 = around 1.001508</output>

<gadget id="calculator">1 - (1_989/1_990)</gadget>
<output>1/1_990 = around 0.000503</output>

<gadget id="calculator">(1_993/1_990) / (1/1_990)</gadget>
<output>1_993</output>

<result>1_993</result>