Improvements to a code! Help!

Question

I have a code and the question is how do i make these three improvements to the code. Here is the code...
import javax.swing.JOptionPane;
public class FibonacciNumber
     public static void main (String[] args)
          String inputString;
          String outputString;
          int previous1;
          int previous2;
          int current = 0;
          int counter;
          int nthFibonacci;
          inputString =
               JOptionPane.showInputDialog("Enter the first "
                              + "Fibonacci number:");
          previous1 = Integer.parseInt(inputString);
          inputString =
               JOptionPane.showInputDialog("Enter the second "
                              + "Fibonacci number: ");
          previous2 = Integer.parseInt(inputString);
          outputString = "The first two numbers of the "
               + "Fibonacci sequence are: "
               + previous1 + " and " + previous2;
          inputString =
               JOptionPane.showInputDialog("Enter the position "
                         + "of the desired number in "
                         + "the Fibonacci sequence: ");
          nthFibonacci = Integer.parseInt(inputString);
          if (nthFibonacci == 1)
          current = previous1;
          else if (nthFibonacci == 2)
          current = previous2;
          else
          counter = 3;
          while (counter <= nthFibonacci)
               current = previous2 + previous1;
               previous1 = previous2;
               previous2 = current;
               counter++;
          outputString = outputString + "\nThe "
                    + nthFibonacci
                    + "th Fibonacci number of "
                    + "the sequence is: "
                    + current;
          JOptionPane.showMessageDialog(null, outputString,
                    "Fibonacci Number",
                    JOptionPane.INFORMATION_MESSAGE);
          System.exit(0);
The three improvements are...
1. check whether the first number entered by the use is less or equal to the second number.
2. check that both numbers are nonnegative.
3. Check whether the user entered a valid value for the position of the desired number in the Fibonacci sequence.
Could someone please help!

Answer 1

So this is how I did it...
     if (previous1 <= previous2)
          System.out.println("Good number choice!")
          else
          System.out.println("Choose a different number!")
          if (previous1 < 0)
               previous1 = -previous1;
          if (previous2 < 0)
               previous2 = -previous2;
          if (nthFibonacci == 1)
          current = previous1;
          else if (nthFibonacci == 2)
          current = previous2;
          else
Is that the correct way? For finding out whether the entered value is okay for the position of the number in the Fibonacci sequence..would I use an IF statement also?