I have a python list containing integers and string data types. All I need to do is to perform a replacement of the element when performing `sum()` function on the list.

For ex:
The list is `lis=[ 1, 4, 'Jan', 8]`, I need to replace `Jan` with integer `1` when performing sum function on it. So the final output of sum function - `sum(lis)` becomes `1 + 4 + 1 + 8` which results to `14`. 

There may be multiple such strings in the list and all those strings needs to be replaced when performing `sum()` on list. Suppose `Feb=2, Mar=3, Apr=4`, `Feb` replaced by `2` when `sum()` is called etc.

I don't want the elements in the list to be replaced. They should be replaced only when the sum is called. Is there any inbuilt function or something to do that?