Passage: In 1813, following Napoleon's defeat in Russia, Frederick William turned against France and signed an alliance with Russia at Kalisz, although he had to flee Berlin, still under French occupation. Prussian troops played a key part in the victories of the allies in 1813 and 1814, and the King himself travelled with the main army of Prince Schwarzenberg, along with Alexander of Russia and Francis of Austria.

Question: What city did Frederick William have to run away from?
Berlin