Conditional Summation

Hi, my name is Jordan.

I have three columns in my spreadsheet. I am checking if values in column A are also values in column B using 

=IF(ISERROR(VLOOKUP(A1,B:B,1,0)),0,IF(VLOOKUP(A1,B:B,1,0),"1",""))

I wish to sum the number of 1's, (or equivalently 'trues') present in column C, one less any given row number that is a multiple of ten.

For example:

1
1
1
0
1
1
1
1
0
1

For example, if the above data set represented C1:C10, the sum would be 7, as opposed to 8.

Not very familiar with Excel, but I assume it will look something like this:

=SUM(C{10k-1}:C1)

Any help will be greatly appreciated,

Thank you.

Jordan H.