18. $A 、 n$ are natural numbers, and
$$
A=n^{2}+15 n+26
$$

is a perfect square. Then $n$ equals $\qquad$
18.23.

Let $A=n^{2}+15 n+26=(n+2)(n+13)$ be a perfect square, we can set $n+2=a^{2}, n+13=b^{2}(a, b$ are positive integers and $b$ $>a)$. Therefore, $11=b^{2}-a^{2}=(b+a)(b-a)$. Then we have $\left\{\begin{array}{l}b+a=11, \\ b-a=1 .\end{array}\right.$ Solving this, we get $\left\{\begin{array}{l}a=5, \\ b=6 .\end{array}\right.$
Thus, $n=a^{2}-2=23$.