Score : 1500 points
Iroha has a sequence of N strings s_1, s_2, ..., s_N.
She will choose some (possibly all) strings from the sequence, then concatenate those strings retaining the relative order, to produce a long string.
Among all strings of length K that she can produce in this way, find the lexicographically smallest one.
The input is given from Standard Input in the following format:
N K s_1 s_2 : s_N
Print the lexicographically smallest string of length K that Iroha can produce.
3 7 at coder codar
atcodar
at
and codar
should be chosen.
3 7 coder codar at
codarat
codar
and at
should be chosen.
4 13 kyuri namida zzzzzzz aaaaaa
namidazzzzzzz
namida
and zzzzzzz
should be chosen.