N = int(raw_input())
dp = [0] * (N + 1)

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