#include <iostream>
#include <vector>
#include <map>
#include <array>

#define REP(i, n) for(int i = 0; i < n; i++)
#define REPR(i, n) for(int i = n - 1; i >= 0; i--)
#define FOR(i, m, n) for(int i = m; i < n; i++)

using namespace std;
template<class T> int maxReturn(T a, T b) {if (a > b) {return a;} else {return b;}}
template<class T> int minReturn(T a, T b) {if (a < b) {return a;} else {return b;}}



int main(void){
    int N;
    cin >> N;
    vector<long> dp;
    
    REP(i, N) {
        if (i == 0 || i == 1) dp.push_back(i+1);
        else dp.push_back(dp[i-1] + dp[i-2]);
    }
    
    cout << dp[N-1] << endl;
}