def find_double_newline(s):
    """Returns the position just after a double newline in the given string."""
    pos1 = s.find(b'\n\r\n')  # One kind of double newline
    if pos1 >= 0:
        pos1 += 3
    pos2 = s.find(b'\n\n')    # Another kind of double newline
    if pos2 >= 0:
        pos2 += 2

    if pos1 >= 0:
        if pos2 >= 0:
            return min(pos1, pos2)
        else:
            return pos1
    else:
        return pos2