In 1960 Random House acquired Alfred A. Knopf. It is believed that the decision to sell was prompted by Alfred A. Knopf, Jr., leaving Knopf to found his own book company, Atheneum Books in 1959. Since its founding, Knopf has paid close attention to design and typography, employing notable designers and typographers including William Addison Dwiggins, Harry Ford, Steven Heller, Chip Kidd, Lorraine Louie, Bruce Rogers, Rudolf Ruzicka, and Beatrice Warde. Knopf books conclude with an unnumbered page titled "A Note on the Type", which describes the history of the typeface used for the book. In addition, Knopf books date the year of the book's current printing on the title page. Knopf published textbooks until 1988, when Random House's schools and colleges division was sold to McGraw-Hill. In 1991, Knopf revived the "Everyman's Library" series, originally published in England in the early 20th century. This series consists of classics of world literature in affordable hardcover editions. The series has grown over the years to include lines of Children's Classics and Pocket Poets. Random House was acquired by Bertelsmann in 1998. In late 2008 and early 2009, the Knopf Publishing Group merged with Doubleday to form the Knopf Doubleday Publishing Group. Random House has been owned since its 2013 merger of Penguin Group by Penguin Random House, a joint venture between Bertelsmann  and Pearson PLC . Many of Knopf's hardcover books are published later as Vintage paperbacks. Vintage Books is a sister imprint of Random House. In 2015, Knopf celebrated its 100th Anniversary by publishing a commemorative book, Alfred A. Knopf, 1915-2015: A Century of Publishing.
Answer this question: How many years after Random House was acquired by Bertelsmann did the Knopf Publishing Group merge with Doubleday?
10