n = int(input())

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