program litere;
var f,g:Text;
    v:array[1..10000]of char;
    n,i,j:integer;
    cont:longint;
begin
assign(f,'litere.in');
assign(g,'litere.out');
reset(f);
rewrite(g);
readln(f,n);
cont:=0;
for i:=1 to n do read(f,v[i]);
for i:=2 to n do
  for j:=i-1 downto 1 do
    begin
      if (v[j]>v[i])then cont:=cont+1;
      if (v[j]='a')then break;
    end;
write(g,cont);
close(f);
close(g);
end.