我需要一些帮助，我想添加一些警告，当我输入一个负数，让我再次键入，但我不知道怎么做。

public class SR {
  public static void main(String[] args) {
    System.out.print("Enter a number: ");
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    System.out.println("The square root of " + n + " is: " + squareRoot(n));
  }

  public static double squareRoot(int num) {
    double t;
    double sqrtroot = num / 2;
    do {
      t = sqrtroot;
      sqrtroot = (t + (num / t)) / 2;
    } while ((t - sqrtroot) != 0);
    return sqrtroot;
  }
}