import string
letters = string.ascii_lowercase
numbers = string.digits


def caesar_cipher(og, shift):
    encrypted = ''
    for i in og.lower():
        let_index = letters.find(i)
        num_index = numbers.find(i)
        if let_index == -1 and num_index == -1:
            encrypted = encrypted + i
        else:
            if let_index == -1:
                encrypted += str((num_index + shift) % 10)
            else:
                new_ix = (let_index + shift) % len(letters)
                encrypted = encrypted + letters[new_ix]
    return encrypted

print(caesar_cipher("abcd", -3))