Scandinavians maintained trade relations and other links with Novgorod from the Viking Age onwards. Merchants from Gotland operated both their own trading house  and the St. Olof church in Novgorod. Scandinavians also carried out isolated  raids on Novgorod. Eiríkr Hákonarson raided  Ladoga in 997, and his brother Sveinn Hákonarson followed suit in 1015. After the marriage of Yaroslav I  to Ingegerd of Sweden in 1019, Ladoga became a jarldom in the orbit of Kievan Rus. It was ruled by Ragnvald Ulfsson, the alleged father of King Stenkil of Sweden . Dynastic marriages took place between Russian and Scandinavian royal families - for example, in  the 1090s Stenkil's granddaughter  Christina married Mstislav of Novgorod, upon whose death in 1132 Novgorod seceded from Kievan Rus. The major turning point into more permanent conflict between Sweden and Novgorod arrived with Sweden's firmer organization into the Catholic Church in the 12th century and papal involvement in crusades against lands controlled by the Orthodox Church.

Based on the above article, answer a question. How many years after Eiríkr Hákonarson raided Ladoga did his brother Sveinn Hákonarson follow suit?
18