#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<random>
#include<stack>
#include<queue>
#include<cassert>
using namespace std;
int inf = 1000000000;

int main(void) {
  long long int n;
  cin >> n;
  vector<long long int> a(51, 0);
  a[1] = 1;
  a[2] = 2;
  a[3] = 3;
  for(int i=3; i<=n; i++){
    a[i] = a[i-1] + a[i-2];
  }
  cout << a[n] << endl;


  return 0;
}