import sys
n=int(sys.stdin.readline().strip())
a=[0]*n
if n>1:
    a[0]=0
if n>2:
    a[1]=1
if n>3:
    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])