The following year, the feud between the nobility and plundering by robber knights continued. In 1385, the castle of Ricklingen, where the Mandelslohs resided, was besieged. When a catapult threw a heavy rock at the troops of Duke Albert, he was hit, and died on 28 June 1385. In the wake of his death, Elector Wenceslas appointed Bernard, his brother-in-law, as co-regent involved him in the government. But his younger brother Henry did not agree with this ruling, and after vain attempts to reach an agreement, the fight flared up again in the spring of 1388. Elector Wenceslas had to assemble an army without the help of Bernard, supported by the town of Lüneburg. From Winsen an der Aller, he wanted to attack Celle, which was held by Henry and his mother. During the preparations, however, Elector Wenceslas fell seriously ill and died shortly thereafter. According to legend, he was poisoned. Lüneburg continued the preparations, formed an alliance with the Bishop of Minden and Count of Schaumburg and set up his own army. On 28 May 1388, battle was joined at Winsen an der Aller; it ended in victory for Henry. According to the provisions of the Treaty of Hanover from the year 1373, after the death of Wensceslas, the Principality passed to the House of Welf. In 1389, a inheritance agreement between the Welfs and the Ascanians was concluded, the treaty of 1374 was abolished, and the Principality was finally secured for the Welfs.
Question: How many years was the treaty of 1374 in effect?
Answer: 15