我的返回负1的方式是错误的，它不会输出负1

Question

public class MathMethods {
public static int GCD (int a, int b){
    int c;
    int d;
    d = -1;
    if (a < 0 || b < 0){
    return d;
    }else{

    while (b!=0){
        c=a%b;
        a=b;
        b=c;

    }
    return a;
    }




}
public static void main (String[] args){
    System.out.println("GCD of 15 and 5: " + MathMethods.GCD(15,5));
    System.out.println("GCD of 12 and 18: " + MathMethods.GCD(12,18));
    System.out.println("GCD of 42 and 56: " + MathMethods.GCD(42,56));
    System.out.println("GCD of 126 and 84: " + MathMethods.GCD(126,84));
    System.out.println("GCD of 72390 and 15000: " + MathMethods.GCD(72390,15000));
}
}

我的GCD代码是正确的，因为我输入的数字会返回一个输出。但是我试图得到一个负数，然后输出负数1，但由于某种原因失败。