Java Roman Numeral to Decimal- special cases i.e. 'CM'

I am having trouble getting the 9 and 4 values( 900,400,90,40,etc)
I think I have the logic correct

Value = Value + 1000('M')
etc..

But I am having trouble getting my else if statements to print 'CM', 'DC' , basically any combination of more than one

Roman Numeral. I am wondering if I use a while loop within the for loop would this be correct?

Here's original homework instruction:

3. Create a program named roman.java. Ask the user to enter a Roman Numeral. Output the numeric value of the Roman Numeral.
Test your program with these numbers: MCMXLII, MCMLXIX, DCCCLXXXVIII
Hint: Create a for loop that steps through each character of roman numeral.

M = 1000, D = 500, C = 100, L = 50, X = 10, V = 5, I = 1.
MDCLXVI = 1000 + 500 + 100 + 50 + 10 + 5 + 1 = 1666

If a digit is out of order, then it is subtracted from the next digit. This is only used for these combinations:
CM = 900, CD = 400, XC = 90, XL = 40, IX = 9, IV = 4


Here's my code:

import javax.swing.*;
import java.util.*;

public class roman
{
   public static void main (String[] args)
   {
    	String S = JOptionPane.showInputDialog(null, "Enter a Roman Numeral:");

	    int Value =0;
	    int[] Letters = {'M','D','C','L','X','V','I'};
	    int[] Numebers = {1000,  500, 100, 50, 10, 5, 1};
		String Numbers = String.valueOf(Letters);

	
		for(int x=0; x<S.length(); x++)
		{
			if (S.charAt(x) == 'M')
				Value = Value + 1000;
			else if (S.charAt(x) == 'D')
				Value = Value + 500;
			else if (S.charAt(x) == 'C')
				Value = Value + 100;
			else if (S.charAt(x) == 'L')
				Value = Value + 50;
			else if (S.charAt(x) == 'X')
				Value = Value + 10;
			else if (S.charAt(x) == 'V')
				Value = Value + 5;
			else if (S.charAt(x) == 'I')
				Value = Value + 1;
		}

	

   		JOptionPane.showMessageDialog(null,"Value of Roman Numeral: " + Value);
   }
}

I have tried many different variations and the above code is only one that compliles?
I would like to know if I am in the right direction and if so, how should I go about
getting the special case values where the Roman Numeral is more than one letter?

I don't want answers~ I would like to better understand what needs to be done next and why- trying to show these guys in my programming class that I am just as capable. /> Thank you.