During the 1960s, the UNC campus was the location of significant political protest. Prior to the passage of the Civil Rights Act of 1964, protests about local racial segregation which began quietly in Franklin Street restaurants led to mass demonstrations and disturbance. The climate of civil unrest prompted the 1963 Speaker Ban Law prohibiting speeches by communists on state campuses in North Carolina.  The law was immediately criticized by University Chancellor William Brantley Aycock and University President William Friday, but was not reviewed by the North Carolina General Assembly until 1965.  Small amendments to allow "infrequent" visits failed to placate the student body, especially when the university's board of trustees overruled new Chancellor Paul Frederick Sharp's decision to allow speaking invitations to Marxist speaker Herbert Aptheker and civil liberties activist Frank Wilkinson; however, the two speakers came to Chapel Hill anyway. Wilkinson spoke off campus, while more than 1,500 students viewed Aptheker's speech across a low campus wall at the edge of campus, christened "Dan Moore's Wall" by The Daily Tar Heel for Governor Dan K. Moore. A group of UNC students along with Aptheker and Williamson filed a lawsuit in U.S. federal court, and on February 20, 1968, the Speaker Ban Law was struck down. In 1968, only a year after its schools became fully integrated, Chapel Hill became the first predominantly white municipality in the country to elect an African American mayor, Howard Lee. Lee served from 1969 until 1975 and, among other things, helped establish Chapel Hill Transit, the town's bus system.
Question: How many total years did Howard Lee serve as Mayor?
Answer: 6