n,a,b  ,  c
1,xy,2,3
2,xy ,2,3
3,xy  ,2,3
4,xy   ,2,3
5,xy,2,3
6, xy,2,3
7,  xy,2,3
8,   xy,2,3
9,xy,2,3
10, xy ,2,3
11,  xy  ,2,3
12,   xy   ,2,3
13,,2,3
14,  ,2,3
15,    ,2,3
16,      ,2,3