Problem:
Score : 200 points

Problem Statement

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 1

3 3
dxx
axx
cxx

Sample Output 1

axxcxxdxx

The following order should be used: axx, cxx, dxx.

Solution in Julia (0.5.0) (judge verdict: Accepted)

MyString=Union{String,SubString{String},Char}
MyStringArray=Union{Array{String,1},Array{SubString{String},1},Array{Char,1}}
parseInt(x::MyString) = parse(Int, x)
parseInt(x::Real) = trunc(Int, x)
parseBigInt(x::MyString) = parse(BigInt, x)
parseBigInt(x::Real) = trunc(BigInt, x)
parseFloat(x::MyString) = parse(Float64, x)
parseFloat(x::Real) = trunc(Float64, x)
parseBigFloat(x::MyString) = parse(BigFloat, x)
parseBigFloat(x::Real) = trunc(BigFloat, x)
parseMap(x::MyStringArray) = map(parseInt, x)
parseMapBig(x::MyStringArray) = map(parseBigInt, x)

function main()
    n,l=parseMap(split(readline()))
    s=sort([chomp(readline()) for i in 1:n])
    println(reduce(*,s))
end
main()