Lithuania became part of the Russian Empire after the final partition of the Polish-Lithuanian Commonwealth in 1795. During World War I Lithuania was occupied by Germany and made part of Ober Ost. On February 16, 1918 the Council of Lithuania declared independence from both Germany and Russia. Three weeks later, the Bolsheviks, encumbered with the Russian Civil War, sued for peace with the Central Powers and signed the  Treaty of Brest-Litovsk. They renounced Russian claims to Finland, Estonia, Latvia, Ukraine, Lithuania and Poland. However, the Lithuanians were only allowed minimal autonomy and could not establish de facto independence. That changed when Germany lost the war and signed the Compiègne Armistice on November 11, 1918. Lithuania soon began organizing basic institutions, and established their first government led by Augustinas Voldemaras. On November 13, 1918, the Soviet Russian government renounced the Treaty of Brest-Litovsk, which had assured Lithuania's independence. The Bolshevik Western Army followed retreating German troops maintaining a distance of about 10-15 kilometres  between the two armies. Demoralized Germans often left valuable armaments and other equipment to the Soviets. The Soviets attempted to spread the global proletarian revolution and sought to establish Soviet republics in the region. They saw Baltic states as a barrier or a bridge into Western Europe, where they could join the German and the Hungarian Revolutions. By the end of December 1918, Bolshevik forces reached eastern Lithuania.
Question: How many years did the two events overlap?
Answer: 15