You will be given a definition of a task first, then some input of the task.
In this task, you're given passages that contain mentions of a time duration related quesry and we are supposed to write answer to a question that involves event “frequency", which refers to how often an event is likely to be repeated. For example, "taking showers" typically occurs ~5 times a week, "going to saturday market" usually happens every few weeks/months, etc. 
 Note that a lot of the questions could have more than one correct answers. We only need a single most-likely answer. Please try to keep your "answer" as simple as possible. Concise and simple "answer" is preferred over those complex and verbose ones.  

Sentence: Boston has a strong mayor – council government system in which the mayor (elected every fourth year) has extensive executive power. Marty Walsh became Mayor in January 2014, his predecessor Thomas Menino's twenty-year tenure having been the longest in the city's history. The Boston City Council is elected every two years; there are nine district seats, and four citywide "at-large" seats. The School Committee, which oversees the Boston Public Schools, is appointed by the mayor. 
Question: How often are elections for mayor held in Boston?
Output:
every fourth year