When Xiaoqiang calculated 8.5 minus a two-digit decimal, he took the minus sign as a plus sign, and the result was 13.09. What is the correct number of this subtraction formula?
小强在计算8.5减去一个两位小数时，把减号当成了加号，算的结果是13.09，这道减法算式的正确得数是多少．
<gadget id="calculator">13.09 - 8.5</gadget>
<output>4.59</output>

<gadget id="calculator">8.5 - 4.59</gadget>
<output>3.91</output>

<result>3.91</result>