from sys import stdin, stdout

input = lambda: stdin.readline()
write = stdout.write

def main():
    N = int(input())

    dp = [0] * (N + 1)
    dp[1], dp[2] = 1, 2
    for i in range(3, N + 1):
        dp[i] = dp[i-1] + dp[i-2]
    print(dp[N])

main()