import java.util.Arrays;
import java.util.Scanner;

public class Yuki057 {
    public static void main(String[] args) {
        Yuki057 hoge = new Yuki057();
    }

    Yuki057() {
        Scanner scanner = new Scanner(System.in);
        int N = scanner.nextInt();

        System.out.println(3.5 * N);

    }

}