How to create a matrix from certain variables

Hello,

if I have set of variables such as:

a0=3 , a1=5 , a2=6 , a3= 9 , a4=2

I want to creat a matrix such as:

n= number of rows,

if I put n=3, the matrix will include a0,a1 and a2 only, the result will be like this:

M=[3

5

6]

if I put n=4, the matrix will include a0,a1,a2 and a3 only, the result will be like this:

M=[3

5

6

9]

Thank you in advance.