#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<stack>
#include<queue>
using namespace std;
int main(void) {
  int n; cin >> n;
  vector<long long> a(n+1, 0LL);
  a[0] = 1; a[1] = 2;
  for(int i=2; i<=n; i++) {
    a[i] = a[i-1] + a[i-2];
  }
  cout << a[n-1] << endl;
  return 0;
}






// EOF