import java.math.*;
import java.util.Scanner;

public class Main {
    static Scanner cin=new Scanner(System.in);
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        while(cin.hasNextInt())
        {
        int n=cin.nextInt(),i;
        if(n==1) {System.out.println("1");continue;}
        if(n==2) {System.out.println("2");continue;}
        BigInteger a,b,c;
        a=BigInteger.valueOf(1);
        b=BigInteger.valueOf(2);
        c=BigInteger.valueOf(0);
        for(i=3;i<=n;++i)
        {
            c=BigInteger.valueOf(0);
            c=c.add(a);
            c=c.add(b);
            a=b;
            b=c;
        //System.out.println(c);
        }
        System.out.println(c);
    }

}
}