var f,g:Text;
    a:array[1..10000] of char;
    c:array[97..122] of integer;
    i,n,j,op:longint;
begin
assign(f,'litere.in');reset(f);
assign(g,'litere.out'); rewrite(g);
readln(f,n);
for i:=1 to n do read(f,a[i]);
for i:=1 to n do begin
  inc(c[ord(a[i])]);
  for j:=ord(a[i])+1 to 122 do
  op:=op+c[j];
end;
write(g,op);
close(g);
end.