Answer -nan [inf] how to solve?

Question

C++ / C#

You need to calculate the geometric mean of the negative elements of the array.
First, I filled the array, counted as many such elements.

double a=0, a1=1;
for (int i=0; i<A;i++){
     if ( arrA[i]< 0) {
         a = arrA[i];
         a1 *= pow(a, 1/k);
}
где k - счётчик отрицательных элементов.

Everything is fine with the loop, but in a1 it prints -nan [ inf ] For example -2 cannot be raised to the power of 0.25...
I would be grateful for your help.

Answer 1

For example -2 cannot be raised to the power of 0.25

And can you?
The geometric mean of negative numbers is not well defined.