import java.util.Scanner;

public class No57 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();

        double expect = 0;

        for (int i = 1; i <= 6; i++) {
            expect += ((double) i) / 6;
        }

        expect *= n;

        System.out.println(expect);

        sc.close();
    }

}