import sys

n = int(input())
l = sys.stdin.read().split()

T = ""
l = sorted(l, key=lambda x: x[0])
while len(l[0]) > 0:
    T += l[0][0]
    l[0] = l[0][1:]
    if "" in l:
        l.remove("")
    if len(l) == 0:
        break
    l = sorted(l, key=lambda x: x[0])
print(T)