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