I have this Java code that works, but I need it to fail if the user enters a letter instead of a number for the first input:

package edu.wit.cs.comp1000;

import java.util.Scanner;

public class PA5b {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

String ch = "";

do {

double avg = getAverage(sc);

System.out.printf("The average is: %.2f%n" , avg);

System.out.println("Do you want to compute another average (y/n)? ");

sc.nextLine();

ch=sc.nextLine();

} while (!ch.equalsIgnoreCase("N"));

}

private static double getAverage(Scanner aSc) {

double sum = 0.00;

double count = 0, num = 0.00;

System.out.println("Enter a stream of non negative numbers (negative number when finished): ");

while (true) {

num = aSc.nextDouble();

if (num < 0)

break;

sum += num;

count++;

}

Float avg = (float) sum / (float) count;

if (count == 0)

return 0.00;

else

return avg;

}