Which one is correct for $\sqrt{-16} \times \sqrt{-1}$? $4$ or $-4$? As we can find in order to evaluate $\sqrt{-16} \times \sqrt{-1}$, we can do it in two ways. 
FIRST
\begin{align*}
     \sqrt{-16} \times \sqrt{-1} &= \sqrt{(-16) \times (-1)}\\
     &= \sqrt{16}\\
     &=4
\end{align*}
SECOND
\begin{align*}
     \sqrt{-16} \times \sqrt{-1} &= \sqrt{16i^2} \times \sqrt{i^2}\\
     &= 4i \times i\\
     &=4i^2\\
     &=-4
\end{align*}
Incidentally if the above is input in  complex mode of Casio scientific calculator, the result comes out as $-4$.
Which of the above solutions is correct?

$$\sqrt a\cdot\sqrt b=\sqrt{ab}$$ only work if $a,b\ge0$
