import java.util.*;
class Main{
  public static void main(String[]a){
    Scanner s=new Scanner(System.in);
    int n=s.nextInt();
    int i=s.nextInt();
    a=new String[n];
    for(i=0;i<n;i++)a[i]=s.next();
    Arrays.sort(a);
    for(i=0;i<n;i++)System.out.print(a[i]);
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz