program litere;
{$APPTYPE CONSOLE}
uses
  SysUtils;

var s,i,j,n,m:integer;
    fin,fout:text;
    v:array [1..10000] of char;
    c,aux:char;
    poz:integer;
{*--------------------------------*}
begin
        assign(fin,'litere.in'); reset(fin);
        assign(fout,'litere.out'); rewrite(fout);

        readln(fin,n); S:=0;
        for i:=1 to n do read(fin,v[i]);

        poz:=0;
        For c:='a' to 'z' do
        begin
                For i:=poz+1 to n do
                begin
                        if v[i] = c then
                        begin
                                inc(poz);
                             //   inc(s);
                            //    if i=poz then dec(s);
                                For j:=i downto poz+1 do
                                begin
                                        v[j]:=v[j-1];
                                        inc(s);
                                end;
                                v[poz]:=c;
                        end;
                end;
        end;

        writeln(fout,s);

        close(fin);
        close(fout);
end.