A cow gives birth to a heifer at the beginning of each year. Every heifer gives birth to a heifer at the beginning of each year from the fourth year. How many years later?

1) Last year's cows are still alive as long as they are not dead this year. 2) How many cows have been born this year? How many cows were there the year before last (n-3 years), so these cows can calve this year, so the litter size is f(n-3). In this way, the number of cattle this year is f(n)=f(n- 1)+f(n-3). Therefore, the recursive formula is obtained: F (1). =4) If 1 year cows can give birth to heifers, it is not difficult to write a program after analysis.