#include <bits/stdc++.h>
using ll = long long;
template <typename T>
constexpr T MOD() { return 1000000007; }
template <typename T>
constexpr T INF() { return std::numeric_limits<T>::max() / 16; }
int main()
{
    int N;
    std::cin >> N;
    std::vector<std::vector<int>> dp(N + 1, std::vector<int>(2, 0));
    dp[0][0] = 1;
    for (int i = 0; i < N; i++) {
        dp[i + 1][0] += dp[i][0] + dp[i][1];
        if (i <= N - 2) {
            dp[i + 2][1] += dp[i][0] + dp[i][1];
        }
    }
    std::cout << dp[N][0] + dp[N][1] << std::endl;
    return 0;
}