N = int(input())
dp = [[0]*2 for i in range(N+1)]

dp[0][0] = 1
for i in range(1,N+1):
    dp[i][0] += dp[i-1][0] + dp[i-1][1]
    if i > 1:
        dp[i][1] += dp[i-2][0] + dp[i-2][1]
print(dp[-1][0] + dp[-1][1])