import java.util.*;
public class Main {
    
    // Normal problem about sorting
    static void solve(){
        Scanner sc = new Scanner(System.in);
        int NOC = sc.nextInt(), L = sc.nextInt(), i = 0;
        String[] s = new String[NOC];
        while(NOC-->0) s[i++] = sc.next();
        Arrays.sort(s);
        for(String a: s) System.out.print(a);
        sc.close();
    }
    
    public static void main(String args[]) {
        solve();
    }
}
/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