var v:array[1..10000] of char;
    c:char;   con,n,i:longint;
    ok:boolean;
    f,g:text;
begin
assign(f,'litere.in'); reset(f);
assign(g,'litere.out'); rewrite(g);
readln(f,n);
for i:=1 to n do begin
inc(n);
read(f,v[n]); end;
i:=1;   con:=0;
repeat  ok:=true;
        i:=1;
        for i:=1 to n-1 do
                  if v[i]>v[i+1] then begin
                                    ok:=false;
                                    c:=v[i];
                                    v[i]:=v[i+1];
                                    v[i+1]:=c;
                                    inc(con);
                                    end;
until ok;
write(g,con);
close(g);
end.