Knopf was founded in 1915 by Alfred A. Knopf Sr. along with Blanche Knopf, on a $5,000 advance from his father, Samuel Knopf. The first office was located in New York's Candler Building. The publishing house was officially incorporated in 1918, with Alfred Knopf as president, Blanche Knopf as vice-president, and Samuel Knopf as treasurer. From the start, Knopf focused on European translations and high-brow works of literature. Among their initial publications were French author Émile Augier's Four Plays, Russian writer Nikolai Gogol's Taras Bulba, Polish novelist Stanisław Przybyszewski's novel Homo Sapiens, and French writer Guy de Maupassant's Yvette, a novelette, and Ten Other Stories. During World War I these books were cheap to obtain and helped establish Knopf as an American firm publishing European works. Their first bestseller was a new edition of Green Mansions, a novel by W. H. Hudson which went through nine printings by 1919 and sold over 20,000 copies. Their first original American novel, The Three Black Pennys by Joseph Hergesheimer, was published in 1917.
Question: How many years did it take Knopf after being founded to become officially incorporated?
Answer: 3; 3