66. Given $O$ is the origin, $P(m, n)(m>0)$ is a point on the function $y=\frac{k}{x}(k>0)$, a line $P A \perp O P$ is drawn through point $P$, and the line $P A$ intersects the positive x-axis at point $A(a, 0)(a>m)$. Let the area of $\triangle O P A$ be $S$, and $S=1+\frac{n^{4}}{4}$. Let $n$ be an integer less than 20, and $k \neq \frac{n^{4}}{2}$, then the minimum value of $O P^{2}$ is $\qquad$
answer: 5