I am supposed to find a series solution for y''=y^2.

My problem is with the y^2. I've started with the following but then I get stuck:

y= sum(anx^n) and y''=sum(n*n-1)anx^n-2 and then I put these into the equation.

With the y^2 I end up with the multiplication of two series...I don't know if this is correct, and I don't know how to deal with the product of two series....