import java.util.*;
class Main{
    public static void main(String args[]){
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        if(n<=0){
            System.out.print;ln("Invalid input");
            return;
        }
        int arr[]=new int[n];
        arr[0]=0;
        arr[1]=1;
        for(int i=2;i<n;i++){
            arr[i]=arr[i-1]+arr[i-2];
        }
        System.out.println(arr[n-1]);
    }
}