def count_min_cost(n, price: list):
c = [float("-inf"), price[1], price[1] + price[2]] + [0] * (n - 2)
for i in range(3, (n + 1)):
c[i] = price[i] + min(c[i - 1], c[i - 2])
return c[n]
n = input(int())
price = [4, 5]
print(count_min_cost(n, price))