S+S-1S+S-2S+S-3S+S-4S+S-5S+S-6S+S-7S+S-8S+S-9S+S-10S+S-11S+S-12S+S-13S+S-14S+S-15S+S-16S+S-17S+S-18S+S-19S+S-20