import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Yuki053 {
    public static void main(String[] args) {
        Yuki053 hoge = new Yuki053();
    }

    Yuki053() {
        double[] dp = new double[101];
        dp[0] = 4;
        dp[1] = 3;

        for (int i = 2; i < 101; i++) {
            dp[i] = (19 * dp[i - 1] - 12 * dp[i - 2]) / 4.0;
        }
        Scanner scanner = new Scanner(System.in);
        int N = scanner.nextInt();
        System.out.println(dp[N]);
    }

}