When the war started, John II had been King of Navarre since 1425 through his first wife, Blanche I of Navarre, who had married him in 1420. When Blanche died in 1441, John retained the government of her lands and dispossessed his own eldest son, Charles , who was made Prince of Viana in 1423. John tried to assuage his son with the lieutenancy of Navarre, but his son's French upbringing and French allies, the Beaumonteses, brought the two into conflict. In the early 1450s they were engaged in open warfare in Navarre. Charles was captured and released; and John tried to disinherit him by illegally naming his daughter Eleanor, who was married to Gaston IV of Foix, his successor. In 1451 John's new wife, Juana Enríquez, gave birth to a son, Ferdinand. In 1452 Charles fled his father first for France, later for the court of his uncle, John's elder brother, Alfonso V at Naples. From 1454 John governed his brother's Spanish realms—the Crown of Aragon—as lieutenant. When Alfonso died in 1458, Charles was arrested and brought to Majorca. John succeeded Alfonso as ruler of the Crown of Aragon. In his will John named Charles as his heir. Among John's early unpopular acts was to quit the war against Genoa, upsetting the merchants of Barcelona. He also refused to aid his nephew, Ferdinand I of Naples, in securing his throne.
Question: How many years ago was when Prince of Viana made compared to when Blanche died?
Answer: 18