Average value of previous 5 samples

bool FirstTime = true;
int AverageOn = 5;
double[] PreviousValues = new double[5];
//A custom equation calculate the averaged values of the previous five readings
public double Ave5Points(double Input)
{
    //this is the first time 
    if (FirstTime)
    {
        FirstTime = false;
        for (int i = 0; i < PreviousValues.Length; i++)
        {
            PreviousValues[i] = Input;
        }
        return Input;
    }
    else
    {
        double average = 0;
        for (int i = 0; i < PreviousValues.Length - 1; i++)
        {
            average += PreviousValues[i];
            PreviousValues[i] = PreviousValues[i + 1];
        }
        PreviousValues[PreviousValues.Length - 1] = Input;
        //average
        average += Input;
        average /= PreviousValues.Length;
        return average;
    }
}