Short Java RndNum

/* @author
 * October 5, 2012
 * High-Low Game part 2
 * Description: Part 1 of the High-low Game. Generates a random integer
 * from 0 to 500 that the user guesses. Outputs "higher" or "lower" until
 * number is guessed correctly.*/
import java.util.*;
public class Expressions{
    public static void main(String[] args){
        Scanner scan_in = new Scanner(System.in);
        int rndNum = 0, guess = 0, numGuesses = 0;
        int ruledOutNumLow = 0, ruledOutNumHigh = 500;
        int rounds = 0, totalGuesses = 0;
        double averageGuessAmount = 0.0;
        String answer = ""; char response;
        boolean validAnswer = false;
        do{
            numGuesses = 0;
            ruledOutNumLow = 0; ruledOutNumHigh = 500;
            rndNum = (int) (Math.random() * 500);
            do{
                System.out.print("Guess a number between 0 and 500, -1 to give up: ");
                guess = scan_in.nextInt();
                if(guess == -1) break;
                if(guess != rndNum){
                    if(guess < rndNum){
                        if(guess < ruledOutNumLow){
                            printThis(ruledOutNumLow, ruledOutNumHigh);}
                        else{
                            ruledOutNumLow = guess + 1;
                            System.out.println("Your guess is too low.");}}
                    else{
                        if(guess > ruledOutNumHigh){
                            printThis(ruledOutNumLow, ruledOutNumHigh);}
                        else{
                            ruledOutNumHigh = guess - 1;
                            System.out.println("Your guess is too high.");}}}
                numGuesses++;}while(guess != rndNum);
            if(guess != -1){
                System.out.println("Congrats! You guessed the number in " + numGuesses + " tries!");
                rounds++;
                totalGuesses += numGuesses;}
            do{
                System.out.print("Play again? (Y/N): ");
                answer = scan_in.next();
                response = answer.charAt(0);
                if(response == 'Y' || response == 'y' || response == 'N' || response == 'n')validAnswer = true;
                if(!validAnswer)System.out.println("Please enter Y or N.");}while(!validAnswer);}while(response == 'Y' || response == 'y');
        averageGuessAmount = (double)totalGuesses/rounds;
        System.out.println("Thanks for playing!");
        System.out.println("You took an average of " + averageGuessAmount + " guesses per round.");}
    public static void printThis(int low, int high){System.out.println("That number has already been ruled out by prior guesses. Only the numbers " + low + " through " + high + " are valid guesses.");}}