def rabbit(n: int): old = [0, 0, 1, 1] new = [1, 1, 1, 2] res = [1, 1, 2, 3] if n > 4: for i in range(4, n): old.append(old[i - 2] + old[i - 1]) new.append(new[i - 1] + old[i] - old[i - 2]) res.append(old[i] + new[i]) return res[n - 1] print(rabbit(int(input())))