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 C (GCC 5.4.1) (judge verdict: Runtime Error)

#include<stdio.h>
int l;
int han(char* a,char* b){
  int i;
  for(i=0;i<l;i++){
    if(a[i]>b[i])return -1;
    if(a[i]<b[i])return 1;
  }
  return 0;
}

int main(){
  int n,j,i;scanf("%d%d",&n,&l);
  char s[100][101];
  for(i=0;i<n;i++){
    scanf("%s",s[i]);
  }
  int ans[100];
  for(i=0;i<n;i++)ans[i]=i;
  for(i=0;i<n;i++){
    for(j=0;j<n;i++){
      if(han(s[ans[i]],s[ans[j]])==-1){
        int tmp=i;
        i=j;
        j=tmp;
      }
    }
  }
  for(i=0;i<n;i++)printf("%s",s[ans[i]]);
  return 0;
}