#include <bits/stdc++.h>

using namespace std;

vector< vector<int> > dp;

int main()
{
    freopen("j.in", "r", stdin);
    int n;
    cin >> n;

    dp.resize(n, vector<int>(2, 0));

    dp[0][0] = 1;
    dp[0][1] = 1;

    for (int i = 1; i < n; ++i)
        dp[i][0] = dp[i - 1][0] + dp[i - 1][1], dp[i][1] = dp[i - 1][0];

    cout << dp[n - 1][0] + dp[n - 1][1];

    return 0;
}