n = int(input())

if n ==1:
    print(0)
elif n == 2 or n==3:
    print(1)
else:
    a = [0]*n
    a[0] = 0
    a[1] = 1
    a[2] = 1
    for i in range(3,n):
        a[i] = a[i-3] + 2*a[i-2] + a[i-1]
    print(a[n-1])