from heapq import *
N = int(input())
S = [list(input())+["{"] for _ in range(N)]

heapify(S)
ans = []
while S:
    x = heappop(S)
    if len(x) > 1:
        ans.append(x[0])
        heappush(S,x[1:])
print("".join(ans))