#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include <utility>
#include <queue>
#include <set>
#include <map>
#include <deque>
#include <iomanip>
#include <cstdio>
#include <stack>
#include <numeric>

using namespace std;
typedef  long long ll;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef vector<VI> VVI;
#define  MP make_pair
#define  PB push_back
#define inf  1000000007
#define rep(i,n) for(int i=0;i<(int)(n);++i)

ll dp[53][2];

int main(){
    int n;
    cin >> n;
    dp[0][0] = 1;
    for(int i=0;i<=n;i++){
        dp[i+1][0] += dp[i][0];
        dp[i+2][0] += dp[i][0];
    }
    cout << dp[n][0]+dp[n][1] << endl;
    return 0;
}