#include<cstdlib>
#include<cstring>
#include<fstream>
#include<cstdio>
using namespace std;
struct yu
{
	char ch[100];
}y[8];
int cmp(const void*a,const void *b)
{
	struct yu *c=(struct yu *)a;
	struct yu *d=(struct yu *)b;
	return strcmp(c -> ch,d -> ch);
}
int main(void)
{
	ifstream fin("substring.in");
	ofstream fout("substring.out");
	int n,i;
	fin>>n;
	for(i=0;i<n;i++)
		fin>>y[i].ch;
	qsort(y,n,sizeof(y[0]),cmp);
	for(i=0;i<n;i++)
		fout<<y[i].ch;
	fin.close();
	fout.close();
	return 0;
}