import java.util.*;
import java.util.function.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        Consumer<Integer> consumer = (num) -> {
        	long[] array = new long[num];
        	for (int i = 0; i < num; i++) {
        		if (i == 0) array[i] = 1;
        		else if (i == 1) array[i] = 2;
        		else array[i] = array[i-1] + array[i-2];
        	}
        	
        	System.out.println(array[num-1]);
        };
        
        consumer.accept(sc.nextInt());
    }
}