Instructions: This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.
Input: Passage: The almost constant wars among the Sumerian city-states for 2000 years helped to develop the military technology and techniques of Sumer to a high level. The first war recorded in any detail was between Lagash and Umma in c. 2525 BC on a stele called the Stele of the Vultures. It shows the king of Lagash leading a Sumerian army consisting mostly of infantry. The infantrymen carried spears, wore copper helmets, and carried rectangular shields. The spearmen are shown arranged in what resembles the phalanx formation, which requires training and discipline; this implies that the Sumerians may have made use of professional soldiers. Question: How many years were the Sumerian city-states at war?
Output:
2000 years