[Java] p-q-Formel | NaN (Not a Number)

import java.io.*;

public class QuadratischeGleichungKoeffizienten{

 public static void main(String [] args) throws IOException{
 bestimmePUndQ();
 }
 
 public static void bestimmePUndQ() throws IOException{
 double p = 0;
 double q = 0;
 BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
 System.out.println("\nWaehlen Sie p:\n");
 p = Double.parseDouble(br.readLine());
 System.out.println("\nWaehlen Sie q:\n");
 q = Double.parseDouble(br.readLine());
 System.out.print("\n\nx1 = " + berechneNullstelleX1(p,q));
 System.out.print("\t x2 = " + berechneNullstelleX1(p,q) + "\n\n");
 System.out.println("\nerneut berechnen? 0/1 (Nein/Ja)");
 int a = Integer.parseInt(br.readLine());
 fortsetzen(a);
 }
 
 public static double berechneNullstelleX1(double p, double q){
 double x1 = 0;
 x1 = 0.5-(p-(Math.sqrt((p*p)-(4*q))));
 return x1;
 }

 public static double berechneNullstelleX2(double p, double q){
 double x2 = 0;
 x2 = 0.5-(p+(Math.sqrt((p*p)-(4*q))));
 return x2;
 }
 
 public static void fortsetzen(int a) throws IOException{
 switch (a){
 case 1 :
 bestimmePUndQ();
 break;
 default :
 System.out.println("\nProgramm beendet...");
 break;
 }
 }
}