import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		
		long dp[] = new long[N+1];
		
		if(N <= 3) {
			System.out.println(N);
		} else {
			dp[2] = 2;
			dp[3] = 3;
			for(int i=4; i<=N; i++) {
				dp[i] = dp[i-2] + dp[i-1];
			}
			System.out.println(dp[N]);
		}
	}
}