import sys
n= int(input())
y = 0 
lst = []
if n == 1:
    y = 0
    print(y)
elif 2<= n <=3:
    y = 1
    print(y)
else:
    a1,a2,a3 = 0,1,1
    for i in range(4,n+1):
        a_nest = a1 + 2*a2 +a3
        a1,a2,a3 = a2,a3,a_nest
    print(a_nest)