<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def y2k(year: int) -> int:
    '''Convert two digit year to four digit year.'''
    assert 0 <= year <= 99, 'Not a two digit year {}'.format(year)
    return year + 1000 if year >= 69 else year + 2000