var v:array[1..60000] of byte;
    s:char;
    f,g:text;
    n,i,aux,k:integer;
    ok:boolean;
begin
assign(f,'litere.in');
assign(g,'litere.out');
reset(f);
rewrite(g);
readln(f,n);
for i:=1 to n do begin read(f,s); v[i]:=ord(s); end;
repeat
ok:=true;
for i:=n downto 2 do if v[i]<v[i-1] then begin
                                      aux:=v[i];
                                      v[i]:=v[i-1];
                                      v[i-1]:=aux;
                                      ok:=false;
                                      k:=k+1;
                                      end;
until ok;
writeln(g,k);
close(f);
close(g);
end.