def f(n):
    if n==1:
        return 1
    elif n==2:
        return 2
    elif n==3:
        return 4
    else:
        return f(n-1)+f(n-2)+f(n-3)
a=int(input())
print(f(a))

看不太懂，哪位好心人能帮我解释一下吗？O(∩_∩)O谢谢