n = int(input())
dp = [[0, 0] for i in range(n+3)] 
dp[0][0] = 1
dp[0][1] = 1
for i in range(n):
    dp[i+1][0] += dp[i][0]
    dp[i+2][1] += dp[i][0]
    dp[i+2][1] += dp[i][1]
print(sum(dp[n]))