n, m = map(int, input().split())
knowntext = input()
ciphertext = input()
plaintext = [""] * m
for i in range(n):
    plaintext[-n + i] = knowntext[i]

for i in range(m - 1, -1, -1):
    char = chr((ord(ciphertext[i]) - ord(plaintext[i])) % 26 + 97)
    if i - n >= 0:
        plaintext[i - n] = char

print("".join(plaintext))