Mathf.Log() with negative numbers?

In the code below I’m doing a Mathf.Log function and i want to get the common power from the variable position (which I am changing in game with a slider). The value is than represented in a UI text object.
Well, everything works fine, just that the LOG function will return NaN when the number is “negative”.

For example:

Mathf.Log(100, 10);
// Returns 2

Mathf.Log(-100, 10);
// Returns NaN

How do I achieve to get a negative log?

using UnityEngine;
using UnityEngine.UI;
using System.Collections;

public class CameraController : MonoBehaviour {

    private float position = 0.0f;
    private float cameraSmooth = 0.01f;
    private Text powerText;

	// Use this for initialization
	void Start () {
        powerText = GameObject.Find("Power").GetComponent<Text>();
	}

    // Update is called once per frame
    void Update()
    {
        powerText.text = Mathf.Log(position, 10.0f).ToString();
        Debug.Log(powerText.text);
    }

    void LateUpdate () {
        transform.position = new Vector3(transform.position.x, transform.position.y, Mathf.Lerp(transform.position.z, position, cameraSmooth));
	}

    public void AdjustPosition(float newPosition)
    {
        position = newPosition;
    }
}

It’s not something that makes any mathematical sense, so you can’t. You can always use positive numbers in Log by using Mathf.Abs to get the absolute value.