How many seasons did Don Knotts play on the Andy Griffith show?

Andy Griffith was the star of the Andy Griffith Show with the character name Andy Taylor. Other cast members included Ron (Ronny) Howard as Opie Taylor; Don Knotts as Deputy Barney Fife; Frances Bavier as Beatrice (Aunt "Bea") Taylor; and later Jim Nabors as Gomer Pyle.