Xiao Ming starts from 1 on the calculator and does continuous addition exercises in the order of natural numbers. When he added to a certain number, the result was 1991, and later he found that a number was missing in the middle, so what is the number that was missing.
小明在计算器上从1开始，按自然数的顺序做连加练习．当他加到某一数时，结果是1991，后来发现中间漏加了一个数，那么，漏加的那个数是多少．
<gadget id="calculator">2_016 - 1_991</gadget>
<output>25</output>

<result>25</result>