def firstDiff(lst1, lst2, i=0):
    if len(lst1) == 0 or len(lst2) == 0:
        return 0
    elif lst1[0] != lst2[0]:
        return i
    else:
        firstDiff(lst1[1:], lst2[1:], i + 1)

list1 = [1, 2, 3, 4, 5]
list2 = [1, 2, 3, 3 ,5]
firstDiff(list1, list2, 0)