{ http://infoarena.ro/problema/litere }
var f,g:text;
    n,i,k,l,aux:integer;
    j:char;
    a:array[1..10000]of char;
    fv:array['a'..'z'] of byte;
begin
     assign(f,'litere.in'); reset(f);
     assign(g,'litere.out'); rewrite(g);
     readln(f,n);
     l:=0;
     for i:=1 to n do
         begin
         read(f,a[i]);
         inc(fv[a[i]]);
         for j:=a[i] to 'z' do
             begin
             if j<>a[i] then l:=l+fv[j];
             end;
         end;
     writeln(g,l);
     close(f); close(g);
end.