creating excel program with C#

I have something like the code below and want to calculate the the mean for the parameter 'value' in column d, using a calculatemean() method that returns a value to the sheet.

please how can i go about it?

static void AddPropertyToWorksheet(float size,string suburb,string city, float value)
{
int row = 2;
Excel.Worksheet Property_info = workbook.Worksheets[1];
while (true)
{
if (Property_info.Cells[row, "A"].Value == null)
{
Property_info.Cells[row, "A"] = size;
Property_info.Cells[row, "B"] = suburb;
Property_info.Cells[row, "C"] = city;
Property_info.Cells[row, "D"] = value;
return;