\begin{multline}
[H_{s,u}]=\sum_{n=1}^{N}\alpha_{n}\sqrt{g_{t}(\phi_{n},\theta_{n},\theta_{tilt})}\\
\times exp(ik\rho(4\lambda(M-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})\\
\times \sqrt{g_{r}(\varphi_{n},\vartheta_{n})exp(ik\rho(4\lambda(n-1)/M)\cos(\phi_{n}-\varphi_{s})\sin\theta_{n})
\end{multline}
Whenever I type this equation, it gives me the following error
Runaway argument?
[H_{s,u}]=\sum _{n=1}^{N}\alpha _{n}\sqrt {g_{t}(\phi _{n},\theta _{\ETC.
Paragraph ended before \multline was complete.
<to be read again>
\par
Kindly help me
}
missing for the final\sqrt{