Can someone help me with my loop code?

A real estate office handles 50 apartment units. When the rent is $600 per month, all the units are occupied. However, for each $40 increase in rent,one unit becomes vacant. Each occupied unit requires an average of $27 permonth for maintenance. How many units should be rented to maximize the profit

import java.util.*;
import java.io.*;

public class ChapterFiveHomework
{
   static Scanner console = new Scanner(System.in);

   public static void main(String[] args) throws FileNotFoundException
   {

   int units;
   double rent;
   double startRent;
   double rentInc;
   double maintenance;
   double totalMaintenance;
   double earnings;
   double earnings2;

   System.out.println("Enter the number of appartment units.");
   units = console.nextInt();

   System.out.println("Enter the rent ammount when all units are occupied.");
   rent = console.nextDouble();

   System.out.println("Enter the increase in rent that results in a vacant"
                                                              +" unit.");
   rentInc = console.nextDouble();

   System.out.println("Enter the ammount to maintain a rented unit.");
   maintenance = console.nextDouble();

   earnings2 = (units-1)*(rent+rentInc)-maintenance*(units-1);
   earnings = units*rent-maintenance*units;

   do
   {
      units--;
      rent = rent+rentInc;
      earnings = (units--)*(rent+rentInc)-(maintenance*(units--));
   }
   while (earnings > earnings2);

   System.out.println("Number of units to rent:" + (units));
   System.out.printf("Ammount to charge for rent: %.2f", rent);

   }
}