Teacher: In this task, you're given a statement, 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'.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Context: During Operation Market Garden, the attempt to seize a bridgehead across the Rhine in the Netherlands, the 704th dropped supplies to allied troops near Nijmegen. Link Information: Operation Market Garden was a failed World War II military operation fought in the Netherlands from 17 to 25 September 1944. Question: When did the operation during which the 704th dropped supplies to allied troops near Nijmegen begin?
Solution: from 17 to 25 September 1944
Reason: The context describes the 704th's actions during Operation Market Garden, and the answer specifies when the operation happened.

Now, solve this instance: Context: Walsh first developed what would become the West Coast offense when he was offensive coordinator of the Cincinnati Bengals, working under legendary head coach Paul Brown. However, Walsh truly perfected this strategy as the head coach of the San Francisco 49ers. Link Information: The team was led in its turnaround from late 1970s doormat by new owner Edward J. DeBartolo Jr. and head coach Bill Walsh. Question: How many years was Walsh the head coach of the team he went to after the Bengals?
Student:
Answer: 18