why my scripts is wrong, i have declare the cost..why i still got
error

// How much it cost for the gas to be consumed per cubic meter
  import java.util.Scanner;

   public class GasCharge
  {
      public static void main(String[] args)
     {
     // Declare constants and variables
        double cost;
        double gas;

     // Read input
        Scanner scn = new Scanner(System.in);
        System.out.print("The amount of gas consumed in cubic
meters:");
        gas = scn.nextDouble();

     // Selection Structure

        if (gas <= 100)
           cost = 5.00;
        else
           if (gas <=300)
              cost = gas * 0.4;
           else
              if (gas <= 500)
                 cost = gas * 0.2;
              else
                 if (gas > 500)
                    cost = gas * 0.1;

     // Display output (result)
        System.out.println ("The total cost is($):" +cost);
     }
  }

ERROR->
GasCharge.java:32: variable cost might not have been initialized
        System.out.println ("The total cost is($):" +cost);
                                                     ^
1 error

----jGRASP wedge2: exit code for process is 1.
----jGRASP: operation complete.

Thanks in advance...=)