Sumproduct across multiple sheets & with multiple conditions!

Hi,

This is the working formula that I have. It works for a single worksheet 'Sheet1'.

=SUMPRODUCT(((Sheet1!$B$51:$B$74)=$C24)*((Sheet1!$C$51:$C$74)=$D24)*(Sheet1!N$51:N$74))

1. I want a similar formula which can scan through multiple sheets say <Sheet1 to Sheet N>
2. In cell $C24 there is a label either "Internal" or "External" | In cell $D24 there is a label either "1" or "2" or "3"
3. I want the sumproduct from Sheet1 to Sheet N, such that ranges $B$51:$B$74 & $C$51:$C$74 satisfy conditions in point 2.
4. Also, range N$51:N$74 contains headcount numbers, meaning my output is an addition of the headcounts through Sheet1 to SheetN satisfying the conditions in point2.

Help would be much appreciated. Thanks in advance to the Excel Gods!

PS: I know there it will require an indirect function, but I cant seem to get it right.

Regards,
Amit G