#include <bits/stdc++.h>

using namespace std;

int N;
long long dp[55];

int main() {
  cin >> N;
  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;

  return 0;
}