w     bags      one optimal packing
0     0         
1     impossible
2     impossible
3     1         3
4     impossible
5     1         5
6     2         3 + 3
7     impossible
8     1         8
9     3         3 + 3 + 3
10    2         5 + 5
11    2         3 + 8
12    4         3 + 3 + 3 + 3
13    2         5 + 8
greedy largest-first on w=12: FAILS, 1 kg left over   (optimal = 4)
greedy largest-first on w=13: 2 bags   (optimal = 2)
