what is the error with .Next() c#

i am writing a game in unity and i want to create one random integer number... i am using the following:

public Random ran = new Random();
public int power = ran.Next(0, 10);

but when i want to run the program it gives me the following error:

Type 'UnityEngine.Random' does not contain a definition for 'Next' and no extension method 'Next' of type `UnityEngine.Random' could be found (are you missing a using directive or an assembly reference?)

Does anyone help me on what is going wrong ???