def fab(n):
    n1 = 1
    n2 = 1
    n3 = 1

    if n<1:
        print("输入有误")
        return -1
    while (n-2)>0:
        n3 = n2 + n1
        n1 = n2
        n2 = n3
        n -= 1

    return n3

result = fab(20)
if result != -1:
    print("总共有%d对小兔崽子诞生！" % result)
    

def fab(n):
    if n<1:
        print("输入有误")
        return -1

    if n == 1 or n == 2:
        return 1
    else:
        return fab(n-1) + fab(n-2)

result = fab(20)
if result != -1:
    print("总共有%d对小兔崽子诞生!" % result)