Kleptography Solution:

n, m = map(int, input().split())
plain = input()[::-1]
ciphertext = list(input()[::-1])
ciphertext_i = 0
full_key = []
while ciphertext_i < n:
    full_key.append((ord(ciphertext[ciphertext_i]) - ord(plain[ciphertext_i])) % 26 + 97)
    ciphertext[ciphertext_i] = plain[ciphertext_i]
    ciphertext_i += 1
while ciphertext_i < m:
    full_key.append( (ord(ciphertext[ciphertext_i]) - full_key[ciphertext_i - n]) % 26 + 97  )
    ciphertext[ciphertext_i] = chr(full_key[ciphertext_i - n])
    ciphertext_i += 1
print(''.join((ciphertext)[::-1]))