I think the formula is just n+1. And since it says that there are 5 poles, we take the first n as 4. For the last n, we take 29 since the bottom row has 30 poles.
Therefore we take th sum of the summation of n=4 to n=29 (n+1).
The answer is 455 poles in total.
Sorry if I wasn't able to give further explanation to this. There are no available equations for sigma here. ^^;