Allowing negative square root

I don't have a clue how to "break" this to actually return a negative square, but that is what I need to figure out. For example, the sqrt of -100 should be -10, but I just get the error telling me that's silly (-1#.IND).

Code:

`#include <iostream>`

#include <cmath>

using namespace std;

double wierdsquareroot(double x)

{

return sqrt(x);

}

int main () // Program calculates the square root of a number

{

double num;

do

{

cout << "Enter a number (a double): "; cin >> num;

cout << "The square root is " << wierdsquareroot(num) << endl;

}while (num != 0);

}