Counting pairs within a collection that meet certain condition(s) using LINQ

I have a very big HashSet of integers.
I want to count the number of any two integers within this HashSet  that have a summation valid within a range of integers.

int bottomB = -100;
int topB = 100;
HashSet<int> nums; // Consider this initialized!
int numberOfValids = nums.Count(/*What lambda goes here?*/); // Using LINQ's Count