# Write a function that takes a list.
#
# For example if the two lists are [1,2,3] and [4,5,6,7],
# your code would return [5, 7, 9, 7]
#
# Do not use any built-in functions except len()

def sum_lists(lst1, lst2):
    result = []

    index = 0
    length = 0

    # because we don't know if the two lists are
    # of the same size, we need to determine
    # how many times we want to repeat
    # (how many iterations needed)

    # to avoid index error (from accessing item
    # outside of the list),
    # let's use the smaller size

    # and then keep the remaining items
    # of the bigger list

    if len(lst1) <= len(lst2):
        length = len(lst1)
        result = lst2
    else:
        length = len(lst2)
        result = lst1

    # repeat until all items of the smaller list
    # are computed
    while index < length:
        result[index] = lst1[index] + lst2[index]
        index += 1

    # leave the remaining of the bigger list
    # in the resultant list

    return result


print(sum_lists([1,2,3], [4,5,6,7]))