Kaden's savings account had $ 707070 in the first year. Each subsequent year, his account accrues interest at 10 \% 10% 10, percent of the balance at the end of the previous year. Let f (n) f (n) f, left parenthesis, n, right parenthesis be the balance of Kaden's account in n ^ \ text {-ta} n -ta n, start superscript, start text, negative, т, а, т, а, end text, end superscript year of saving.