var f1,f2:text;
    i,n,nr:longint;
    ok:boolean;
    c:array[1..10010] of char;
    aux:char;
begin
        assign(f1,'litere.in');
        reset(f1);
        assign(f2,'litere.out');
        rewrite(f2);
        readln(f1,n);
        for i:=1 to n do
                read(f1,c[i]);
        while ok=false do
        begin
                ok:=true;
                for i:=1 to n-1 do
                        if c[i]>c[i+1] then
                        begin
                                aux:=c[i];
                                c[i]:=c[i+1];
                                c[i+1]:=aux;
                                ok:=false;
                                inc(nr);
                        end;
        end;
        writeln(f2,nr);
        close(f1);
        close(f2);
end.