#include <iostream>
#include <vector>
#include <cmath>
#include <iomanip>
#include <array>
#include <tuple>
#include <algorithm>
#include <map>
#include <stack>
#include <queue>
#include <set>
#include <list>
#include <cstdio>
#include <limits.h>
#include <string.h>

using namespace std;
using ull = unsigned long long;
using ll = long long;
using lli = long long int;
using ld = long double;
using pa = pair<int,int>;
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define GRESORT(v) sort(v.begin(),v.end(),greater<int>())
#define REVERSE(v) reverse(v.begin(),v.end())
#define overlap(v) v.erase(unique(v.begin(),v.end()),v.end())
#define debug(x) cout << #x << ": " << x << endl
#define INF 999999999

//4近傍
int dy[]={0, 0, 1, -1};
int dx[]={1, -1, 0, 0};

int n;

void input(){
    cin >> n;
}

lli dp(){
    vector<lli> data(60);
    data[0] = 0;
    data[1] = 1;
    data[2] = 2;
    data[3] = 3;
    for(int i = 4; i <= n; i++){
        data[i] = data[i-1] + data[i-2];
    }
    return data[n];
}

int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    input();
    cout << dp() << endl;

    return 0;
}