import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    // dp[i]はちょうどiマスに行くパターン数
    long[] dp = new long[N + 1];
    dp[1] = 1;
    dp[2] = 2;
    for(int i = 3; i < N + 1; i++) {
      dp[i] = dp[i - 1] + dp[i - 2];
    }
    System.out.println(dp[N]);
  }
}