SQL & PL/SQL

I think there should be a way to solve the following problem with analytic functions but I don't see the solution right now :-(
Maybe someone can help me. I have a table like this

create table demo
(n number,
 t varchar2(1));
 
insert into demo values (1,'A');
insert into demo values (2,'A');
insert into demo values (3,'B');
insert into demo values (4,'C');
insert into demo values (5,'C');
insert into demo values (6,'C');
insert into demo values (7,'A');
insert into demo values (8,'A');
insert into demo values (9,'B');
insert into demo values (10,'C');

and I want to get the row_number within a group of equal letters in column t, ordered by column n, meaning that counting should start with 1 again, when 'A' appears once more after there where other letters between.
When I use something like row_number() over (partition by t order by n) counting the A's continues with 3, where it should start with 1 again and so on

select 
   n, 
   t,
   row_number() over (partition by t order by n) v
from demo
order by n;

N  T V Should be 
-- - - ---------
1  A 1         1
2  A 2         2
3  B 1         1
4  C 1         1
5  C 2         2
6  C 3         3
7  A 3         1
8  A 4         2
9  B 2         1
10 C 4         1