program litere;
var f,g:Text;
    v:array[1..10000]of 0..26;
    n,i,j:integer;
    cont:longint;
    x:char;
begin
assign(f,'litere.in');
assign(g,'litere.out');
reset(f);
rewrite(g);
readln(f,n);
cont:=0;
read(f,x);
for i:=2 to n do
  begin
    read(f,x);
    for j:=ord(x)-ord('a')+1 to 25 do
    cont:=cont+v[j];

    v[ord(x)-ord('a')]:=v[ord(x)-ord('a')]+1;
  end;

{read(f,v[1],v[2]);
if (v[2]<v[1])then cont:=cont+1;
for i:=3 to n do
  begin
    read(f,v[i]);
    for j:=i-1 downto 1 do
      if (ord(v[j])>ord(v[i]))then cont:=cont+1;
  end; }
write(g,cont);
close(f);
close(g);
end.