Context: On 6 and 13 December 1971, the Soviet Navy dispatched two groups of cruisers and destroyers and a submarine armed with nuclear missiles from Vladivostok; they trailed US Task Force 74 into the Indian Ocean from 18 December 1971 until 7 January 1972. The Soviets also had a nuclear submarine to help ward off the threat posed by USS Enterprise task force in the Indian Ocean.

Question: How many days passed between the first and second group of vessels sent by the Soviet Navy to trail US Task Force 74?

Answer:
7