def check_correct(string, alphabet):
    """Check string in alphabet"""
    for c in string:
        if c not in alphabet:
            raise "Wrong data: found wrong characters"
 
def code(char, alphabet):
    """Return number of character in alphabet"""
    return alphabet.find(char)

def shift_char(char1, char2, alphabet):
    """Shift character with the other one"""
    c = code(char1, alphabet) + code(char2, alphabet)
    if c > len(alphabet):
        c -= len(alphabet)
    return alphabet[c]
  
def unshift_char(char1, char2, alphabet):
    """Unshift character with the other one"""
    c1 = code(char1, alphabet)
    c2 = code(char2, alphabet)
    if c1 < c2:
        c1 += len(alphabet)
    c = c1 - c2
    return alphabet[c]
  
def encrypt(phrase, autokey, alphabet):
    """Encrypt data with autokey"""
    check_correct(phrase, alphabet)
    check_correct(autokey, alphabet)
    key = autokey + phrase
    encrypted = ""
    for i in range(len(phrase)):
        encrypted += shift_char(phrase[i], key[i], alphabet)
    return encrypted

def decrypt(encrypted, autokey, alphabet):
    """Decrypt data with autokey"""
    check_correct(encrypted, alphabet)
    check_correct(autokey, alphabet)
    key = autokey
    for i in range(len(encrypted)): 
        key += unshift_char(encrypted[i], key[i], alphabet)
    return key[len(autokey):]