alphabet = "яабвгдеёжзийклмнопрстуфхцчшщъыьэю"

def sub(s1, s2):
    i = 0
    result = ""
    while i < len(s1):
        code = alphabet.index(s1[i]) - alphabet.index(s2[i])
        if code < 0:
            code = code + 33
        result += alphabet[code]
        i += 1
    return result

if __name__ == '__main__':
    first = "ёлисувшоюцомювызпэъмо"
    second = "ъэлвшреээтжщюигвфбсцх"
    third = "жъыхйтсжыашшьямышзьвг"

    k_ch = sub(second, first)
    plain = sub(third, k_ch)
    print(plain)