def solve(n):
    dp = [0] * max(4, n + 1)
    dp[1] = 1
    dp[2] = 2
    dp[3] = 3
    for i in range(4, n + 1):
        dp[i] = dp[i - 1] + dp[i - 2]
    return dp[n]


def main():
    n = int(input())
    print(solve(n))


if __name__ == "__main__":
    main()