If your numbers are in range A1:A100, use below formula

=SUMPRODUCT((A1:A100)*(MOD(ROW(A1:A100)-ROW(A1)+1,2)=0))

Above formula is for every 2nd row. Replace 2 with N. Hence, for every 5th row -

=SUMPRODUCT((A1:A100)*(MOD(ROW(A1:A100)-ROW(A1)+1,5)=0))

This is a generic formula and will work for any range. If you range is B7:B50, your formula would become

=SUMPRODUCT((B7:B50)*(MOD(ROW(B7:B50)-ROW(B7)+1,2)=0))