单元格/范围作为函数的参数，在VBA中计算平均值

I'm trying to recreate the sum function that adds up values within a range, I also want it calculate the average, I will add more calculations later on but so far I'm only testing out the sum portion before doing the average. But I'm not sure how to pass the cells as a parameter into the function, also not sure on how to take the average of the sum since I don't know how to count the number of cells in between the two given cells in the parameter.I'm trying to recreate the sum function that ad