How to get sum of linq count results - linq

Question

I have a database with peoples names and ages amongst other things. I have a query which groups by names and gets amount of each name matching the criteria. The ListHelper type is only a class containing the two properties.
IEnumerable<ListHelper> HelperEnumerable = null
HelperEnumerable = _repository.Persons
.Where(b => b.Age < 18)
.GroupBy(
n => n.FirstName
, a => a.FirstName
, (key, count) => new ListHelper { Name = key, Amount = count.Count() }
);
When I ToList() the HelperEnumerable the result is like:
Name: "Michael", Amount: 100,
Name: "Eva", Amount: 122,
Name: "Lisa", Amount: 71,
etc
How can i get a similar result but with count of all persons matching the criteria with a result like this:
Name: "All", Amount: 17280
I would like to have the key value pair so all the rest of the code could stay the same, only this query would return the count of all matchig rows instead of grouped by any particular columm.
I've tried this which returns only the int count:
HelperEnumerable = _repository.Persons
.Where(b => b.Age < 18).Count();
And I can't add a
.Select(a => (key,count) new ListHelper{ key = "All", count = a })
after a Count() to try to project the result to have two fields.

Answer 1

What does:
IEnumerable<ListHelper> HelperEnumerable = null
HelperEnumerable = _repository.Persons
.Where(b => b.Age < 18)
.GroupBy(
n => "All"
, (key, count) => new ListHelper { Name = key, Amount = count.Count() }
);
Not do that you need it to do?
or why not just:
new ListHelper{ key = "All", count = _repository.Persons.Where(b => b.Age < 18).Count() };
??