N = int(input())
dp = [0] * (N + 1)
dp[0] = 1
ans = 0
for i in range(N):
    dp2 = [0] * (N + 1)
    for j in range(N + 1):
        if j > 1:
            dp2[j] += dp[j - 2]
        if j > 0:
            dp2[j] += dp[j - 1]
    dp = dp2
    ans += dp[N]

print(ans)