I have two nested lists like those
{{a,A},{b,B},{c,C}...{z,Z}}
{{a,1},{c,2}...{t,12}}.
I want to combine them to get a new list that looks like this
{{a,A+1},{c,C+2}...{t,T+12}}
where the first element of each sublist exists in both list 1 and list 2 and the lists are of unequal length. How would I do this?