def check_repeated(str1, str2):
    """
    Given 2 string check if these strings have a diff of 1 character
    if it's the case return the position of the character otherwise return None
    """
    assert len(str1) == len(str2)
    position = None
    quantity = 0
    for i in range(len(str1)):
        if str1[i] != str2[i]:
            quantity += 1
            position = i
    if quantity == 1:
        return position
    else:
        return None