import java.util.*;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        int N = scan.nextInt();
        if (!match(N)) System.exit(1);
        System.out.println(N * 3.5);
    }

    public static boolean match (double... args) {
        for (double i : args) {
            if (i < 1||i > 1000000) return false;
        }
        return true;
    }
}