Mathf.Abs but for a double?

So I’m making an incremental game and I chose to use doubles to store my big fat numbers because floats and ints got capped far too soon for the numbers I’m going to end up using towards the end game.
I’ve run into an issue in my “currency converter” code as I call it for formatting the double to look nice on display, however I need the absolute value because sometimes the number being run through will be a negative. And as you can see my “check” requires the number to be positve… Mathf.Abs doesn’t work with doubles

	public string ConvertDoubles(double convert){
		if (scientificNotation == false) {
			for (int i = 0; i < valueCheck.Length; i++) {
				convert = Mathf.Abs (convert);
				if (convert >= valueCheck *) {*

write your own Absolute method it is far from complex math function like this:

public static double AbsoluteValueOf(double number)
{
          double num = number;           
          if(number<0)
                  num = -1*number;        //there are other ways to do that, maybe more efficient but more complex
           return num;
}

or even better use the one C# provides Math.Abs(), will require to add on the script using System;