Carlos,
the problem does not appear very well defined. If L is greater than 1,
say 4, then (n-4)! is not defined for n=1, 2, 3, because it is
negative.

At any rate, you can approach this problem by breaking it to columns.
$G$1 holds L.

One helper column (say A:A, starting from A1) should have the numbers
1, 2, ...

The next one should have the numerator =(1+$G$1)*exp(A1)

The third column should contain the factorial. A quick way to do this
is with an array formula =PRODUCT(ROW(INDIRECT("1:" & V), where V is
the expression that holds your number. In this case V would be A1-$G$1.
As you can see this last one will product negative values and #REF! in
the cell.

Continuing, the next column would be =B1/C1

Finally, the last column would be =SUM($D$1:D1) and copied. At each row
you have the result for the corresponding K.

I suspect this is not quite what you need, maybe other contributors can
help, as surely will your clarification on the problematic factorial.

HTH
Kostis Vezerides