How to create a vector repeating different numbers with specific amounts?

If I have this matrix:

M=[1    3
   2    4
   3    1
   4    2
   5    1];

How could we obtain a vector x which repeats each entry of M(:,1) the number of M(:,2) entries obtaining:

x=[1
   1
   1
   2
   2
   2
   2
   3
   4
   4
   5];

Answer 1

[OCDER]

Is this homework? Hint: use repelem

https://www.mathworks.com/help/matlab/ref/repelem.html