>>> from stop import stop

>>> stop (1, 'N')
1
>>> stop (1, 'S')
5
>>> stop (2, 'N')
4
>>> stop (2, 'S')
9
>>> stop (3, 'N')
12
>>> stop (3, 'S')
2
>>> stop (4, 'N')
10
>>> stop (4, 'S')
7
>>> stop (2, 'K')
0
>>> stop (7, 'N')
0