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;
     }
 }