Matrix multiplication is associative and MCS ( matrix chain multiplication ) uses the following matrices:
$\begin{array} \text{M1} & 10^* 100 \\ M2 & 100^* 5 \\ M3 & 5^* 50 \\ M4 & 50^* 1 \end{array}$
The number of orderings that are possible to compute $M1 \ M2 \ M3 \ M4$ are _________.