If the product of n matrices $\left[\begin{array}{ll}1& 1\\ 0& 1\end{array}\right]\left[\begin{array}{ll}1& 2\\ 0& 1\end{array}\right]\left[\begin{array}{ll}1& 3\\ 0& 1\end{array}\right]\dots \cdots \left[\begin{array}{ll}1& n\\ 0& 1\end{array}\right]$ is equal to the matrix $\left[\begin{array}{cc}1& 378\\ 0& 1\end{array}\right]$ then the value of n is equal to-