Q: In this task, you're given a passage, further information available on a particular linked term from the statement, and a question. Your job is to generate the answer to the question by using the information provided. If there is no clear answer obtainable, output 'none'.
Passage: In Roman times, it was a village in the province of Moesia, first mentioned in 169. After the decline of the Roman Empire, the town turned out to be located at the northern border of the Byzantine Empire. In 1059, it was named Nicopolis, Greek for "City of Victory". During most of the Middle Ages, it was part of the Bulgarian Empire from its foundation in 681. After the fall of Tarnovo in 1393, the last Bulgarian Tsar Ivan Shishman defended what remained of the Empire from the fortress of Nikopol, where he was captured after the town was conquered by the Ottomans in 1395. Nikopol is therefore sometimes considered the capital of Bulgaria during these two years. It was the site of the Battle of Nicopolis, the last large-scale crusade of the Middle Ages, in 1396. At the fortress of Nicopolis, the united armies of Christian Europe headed by Hungarian king Sigismund and various French knights were defeated by the Ottomans under Bayezid I and his Serbian vassal Stefan Lazarevic.
Link Information:

Ivan Shishman () ruled as emperor (tsar) of Bulgaria in Tarnovo from 1371 to 3 June 1395. Question: How long did the last Bulgarian Tsar rule the Empire?
A:
Answer: 24