#include <bits/stdc++.h>

using namespace std;

using int64 = long long;


int main() {
  int N;
  cin >> N;
  int64 v[52];
  v[1] = 1;
  v[2] = 2;
  for(int i = 3; i <= 50; i++) {
    v[i] = v[i - 2] + v[i - 1];
  }
  cout << v[N] << endl;
}