An extra -s or -es is added to masculine and neuter nouns in the genitive.
-es is often added to nouns with only one syllable: das Buch – der Titel des Buches/der Titel des Buchs
-es is always added to nouns ending with -s, -ß, x, – z, -tz: das Glas – die Farbe des Glases
-s is added to nouns with more than one syllable: das Hotel – der Name des Hotels