import java.util.*;

public class Main {
	
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		double a[]=new double[105];
		a[0]=4;
		a[1]=3;
		for(int i=2;i<105;i++){
			a[i]=(a[i-1]*19-a[i-2]*12)/4;
		}
	System.out.println(a[n]);
		
	}}