Given: 1 -> 2 -> 3 + 4 -> 5 -> 6. Expected output: 5 -> 7 -> 9

file.py

def add(li1, li2):
    sum = []
    while li1 and li2:
        sum.append(li1.pop(0) + li2.pop(0))
    while li1:
        sum.append(li1.pop(0))
    while li2:
        sum.append(li2.pop(0))
    return sum
    
string = '(1 -> 2 -> 3) + (3 -> 4 -> 5)'

sums = []
for term in string.split('+'):
    term = term.replace('(', '')
    term = term.replace(')', '')
    nums = []
    for num in [s.strip() for s in term.split('->')]:
        nums.append(int(num))
    sums = add(sums,  nums)

sums = map(str, sums)
sums = ' -> '.join(sums)
print(sums)