Finding the sum of the components of a series.

I was reading sequence and series and there I saw the formula for the sum of the squares of the natural numbers ($1^2+2^2+3^2+\cdots+n^2$) so I just incurred a doubt about the sum obtained when these squared numbers are inversed i.e $$\frac{1}{1^2}+\frac{1}{2^2}+\frac{1}{3^2}+\cdots+\frac{1}{n^2}$$ I tried a lot but did not reach a conclusion. I tried to convert the whole sum of the components into a single fraction, tried to take commons etc. but did not succeed. So is there a way to find a general form for the sum ? If yes please help me out. Any help is appreciated. $$Thank you$$