Add elements of a list to the last elements of sublists

Question

I have two lists, where the number of elements of list 1 = number of sublists of list 2:

list1 = {a,b};
list2 = {{f,g,h,i},{j,k,l,m}}

I would like to add elements of list1 to the last elements of list2, so I did

list3 = MapThread[Append,{list2,list1}];

list3 = list3 /. {x1_,x2_,x3_,x4_,x5_} -> {x1,x2,x3,x4+x5}

It works but I suspect there is a more efficient way with, perhaps, MapAt or some combination of functions that I can't figure out. How would you do it? Thanks in advance.

Answer 1

With padding:

list2 + Map[PadLeft[{#},4]&, list1]

{{f, g, h, a + i}, {j, k, l, b + m}}

Answer 2

See code below:

list2[[;; , -1]] = list2[[;; , -1]] + list1;
list2

{{f, g, h, a + i}, {j, k, l, b + m}}

EDIT: or the more compact, suggested by @Henrik Schumacher in the comment below:

list2[[;; , -1]] += list1