n = int(input())
dp = [0 for _ in range(n + 1)]
dp[0] = 1

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

print(dp[-1])