题解 | 牛牛学数列6

n = int(input())
An = []
for i in range(1,n+1):
    if i==1:
        An.append(0)
    elif i in {2,3}:
        An.append(1)
    else:
        An.append(An[i-4]+2*An[i-3]+An[i-2])
print(An[n-1])