How to add an integer in a string, java and limit user input

Hi I needed help with doing a calculation in this right here. On the line where it has single, i want to add the number to the calculation but it just adds it as if im just writing the number itself in a sentence. How do i add it as if its a calculation, im very stuck and need help. (Its the last line of code). I also wanted to know how i can limit the amount of letters a user can input, since i only want them to either enter 's' or 'm'. How can i limit them to only these two so they dont use a letter like 'g' for example, since that wont work.

import java.util.Scanner; 
public class FedTaxRate 
{
 public static void main(String[] args)
 {
  String maritalStatus; 
  double income; 
  int single = 32000; 
  int married = 64000;

  Scanner status = new Scanner(System.in);
  System.out.println ("Please enter your marital status: "); 
  maritalStatus = status.next();


  Scanner amount = new Scanner(System.in);
  System.out.print ("Please enter your income: ");
  income = amount.nextDouble();

  if (maritalStatus.equals("s") && income <= 32000 )
  {
     System.out.println ("The tax is " + income * 0.10 + ".");
  }
  else if (maritalStatus.equals("s") && income > 32000) 
  {
     System.out.println ("The tax is " + (income - 32000) * 0.25 + single + ".");
  }

  }
 }