Judge Valid

    public static void main(String[] args) {

        Scanner scan = new Scanner(System.in);

        double digit = Double.parseDouble(scan.nextLine());
        String input = scan.nextLine();
        String output = scan.nextLine();

        switch (input) {
            case "mm":
                digit = digit / 1000;
                break;
            case "cm":
                digit = digit / 100;
                break;
        }
        switch (output) {
            case "mm":
                digit = digit * 1000;
                break;
            case "cm":
                digit = digit * 100;
                break;
            case "m":
                digit = digit * 1;
                break;
        }
        System.out.printf("%.3f", digit);
    }