import java.util.*;
public class main
{
    public static void main(String[]args){
        Scanner sc=new Scanner(System.in);
        if(!sc.hasNextInt()){
            System.out.println("Invalid input");
            return;
        }
        //int n=sc.nextInt();
        int first=0;
        int second=1;
        int next=first+second;
        if(n<1){
            System.out.println("invalid input");
            return;
        }
        for(int i=2;i<n;i++)
        {
            next=first+second;
            first=second;
            second=next;
        }
        System.out.println(next);
    }
}