var v:array[1..60000] of byte;
    s:char;
    f,g:text;
    n,i,aux,k,j: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;
for i:=2 to n do begin
aux:=v[i];
j:=i-1;
ok:=false;
while (j>=1) and (not ok) do
if aux<v[j] then begin k:=k+1; v[j+1]:=v[j]; j:=j-1; end
            else ok:=true;
            v[j+1]:=aux;
                                end;
writeln(g,k);
close(f);
close(g);
end.