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.
-----Constraints-----
 - 1 ≦ N, L ≦ 100
 - For each i, the length of S_i equals L.
 - For each i, S_i consists of lowercase letters.
-----Input-----
The input is given from Standard Input in the following format:
N L
S_1
S_2
:
S_N
-----Output-----
Print the lexicographically smallest string that Iroha can produce.
-----Sample Input-----
3 3
dxx
axx
cxx
-----Sample Output-----
axxcxxdxx
The following order should be used: axx, cxx, dxx.
n, l = map(int, input().split())
str = [input() for i in range(n)]
str.sort()
for i in str:
    print(i, end = '')
n, l = map(int, input().split())
str = [input() for i in range(n)]
str.sort()
for i in str:
    print(i, end = '')