# https://www.codewars.com/kata/5324945e2ece5e1f32000370/

def sum_strings(s1, s2):
    res = ""
    carry = 0
    s1, s2 = s1.zfill(len(s2)), s2.zfill(len(s1))
    for d1, d2 in zip(s1[::-1], s2[::-1]):
        carry, total = divmod(int(d1) + int(d2) + carry, 10)
        res += str(total)

    return (str(carry) + res[::-1]).lstrip('0') or '0'