a,b,c,d,e,f
1,2,3,4,5,6
2,3,4,5,6,7
3,4,5,6,7,8
4,5,6,7,8,9
5,6,7,8,9,10
6,7,8,9,10,11
7,8,9,10,11,12
8,9,10,11,12,13
9,10,11,12,13,14
10,11,12,13,14,15
11,12,13,14,15,16
12,13,14,15,16,17
13,14,15,16,17,18
14,15,16,17,18,19
15,16,17,18,19,20
16,17,18,19,20,21