n=int(input())
if n==1:
    print(0)
elif 1<n<4:
    print(1)
else:
    a1,a2,a3=0,1,1
    for i in range(4,n+1):
        s=a1+2*a2+a3
        a1,a2,a3=a2,a3,s
    print(s)