import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int n = scanner.nextInt();
		long dp[]=new long[n+1];
		dp[0]=0;
		dp[1]=1;
		dp[2]=2;
		for(int i=3;i<=n;i++){
			dp[i]=dp[i-1]+dp[i-2];
		}
		System.out.println(dp[n]);
	}
}