题解 | #统计每个月兔子的总数#

n=int(input())
a1=1
a2=0
a3=0
if n==1:
    print(a1)
else:
    for i in range(2,n+1):
        a3+=a2
        a2=a1
        a1=a3
print(a1+a2+a3)