import java.util.*;
import java.util.Arrays;

class Main{
	static int n;
	static long memo[] = new long[51];

	static long saiki(int now){
		if(now == n) return 1;
		if(now >= n) return 0;
		if(memo[now] != -1) return memo[now];

		long sum = 0;
		sum += saiki(now + 1);
		sum += saiki(now + 2);
		return memo[now] = sum;
	}

	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		n = sc.nextInt();
		Arrays.fill(memo,-1);
		System.out.println(saiki(0));
	}
}