Adding an IF function to the current formula?

[chaskin]

I am using the current formula to automatically find value for a particular name and add them together to find the average. =(VLOOKUP(A1,Sheet1!A:D,4,0)+INDEX(Sheet2!D:D,MATCH(A1,Sheet2!A:A,0)))/2

This final number gets imported to the sheet titled "Combined"

When the name being looked up is present in both sheet 1 and sheet 2, this formula works flawlessly. The issue I have is not every name appears in both sheets. This returns a value of #N/A. I am wondering if there is another function that could be added so that if the name only appears in either sheet 1 or sheet 2, but not both, the formula would simply return the value in the sheet where the name does appear. If the name does not appear in either sheet, I am ok with it returning #N/A as I can easily filter those out of the workbook.

I have attached the worksheet for reference. Thanks in advance!