Takuya's parents gave him $100 as a birthday gift. Since he loves board games, he spent $20 out of his present on board games at the end of each month until his money ran out. Let f(n) be the amount of money that remained from Takuya's present during month n, where n=1 represents the month he received his present. f is a sequence. What kind of sequence is it?

f(1)=
f(n)=f(n−1)