Year	Number
1960	-6
1961	-6
1962	-6
1963	-6
1964	-6
1965	-6
1966	-7
1967	-7
1968	-7
1969	-7
1970	-7
1971	-7
1972	-8
1973	-8
1974	-8
1975	-8
1976	-8
1977	-8
1978	-8
1979	-8
1980	-8
1981	-8
1982	-7
1983	-8
1984	-8
1985	-8
1986	-8
1987	-8
1988	-8
1989	-8
1990	-8
1991	-8
1992	-4
1993	-4
1994	-4
1995	-4
1996	-4
1997	-4
1998	-4
1999	-4
2000	-4
2001	-4
2002	-4
2003	-4
2004	-4
2005	-4
2006	-4
2007	-4
2008	-4
2009	-4
2010	-4
2011	-4