Iroha has a sequence of N strings S_1, S_2, ..., S_N. The length of each string is L.  She will concatenate all of the strings in some order, to produce a long string.  Among all strings that she can produce in this way, find the lexicographically smallest one.  Here, a string s=s_1s_2s_3...s_n is _lexicographically smaller_ than another string t=t_1t_2t_3...t_m if and only if one of the following holds:    * There exists an index i(1≦i≦min(n,m)), such that s_j = t_j for all indices j(1≦j<i), and s_i<t_i.   * s_i = t_i for all integers i(1≦i≦min(n,m)), and n<m.
n,l = map(int, input().split())
print(sorted([input() for i in range(n)]))
n,l = map(int, input().split())
list = sorted([input() for i in range(n)])
print(*list, sep='')