#include <bits/stdc++.h>
using namespace std;

int main() {
  int N;
  cin >> N;
  vector<long long> dp(N + 1);
  dp[1] = 1, dp[2] = 2;
  for (int i = 3; i <= N; ++i)
    dp[i] = dp[i - 1] + dp[i - 2];
  cout << dp[N] << endl;
}