Why do I get Inf+ Nani?
Because of
n=sn;
Try each step directly at the command line and see what happens...
You need to set a value for the total of the summation of each term initially to zero and then accumulate the terms into that variable -- sn is ok for the name but you don't accumulate a sum of the terms in n, you replaced in with the individual term going forward.
To get the subsequent terms you need to increment n, that is correct --
n=n+1;
where the n=sn; line is instead; the implementation of accumulating the summation is left for you to consider how to do that a little more...
