var {s:string;}
    s:array[1..10000] of char;
    n,i,k,m,p,j:integer;
    f,g:text;
    c,x:char;
begin
assign(f,'litere.in');
assign(g,'litere.out');
reset(f);
rewrite(g);
readln(f,n);
for i:=1 to n do read(f,s[i]);
p:=1;
for c:='a' to 'z' do
 begin
  k:=0;
  for i:=1 to n do if s[i]=c then inc(k);
  if k<>0 then
  for i:=1 to n do if s[i]=c then
  begin
   m:=m+(i-p);
   if (i<>1) and (s[p]<>c) then
   for j:=i downto p+1 do begin
                    x:=s[j];
                    s[j]:=s[j-1];
                    s[j-1]:=x;
                    end;
   inc(p);
 end;
 end;
write(g,m);
close(f);
close(g);
end.