def ctoi(c):
	return ord(c) - ord("a")
	
def itoc(i):
	return chr(ord("a") + i)

def deg(s):
	res = [0] * 26
	for c in s:
		res[ctoi(c)] += 1
	return tuple(res)
	
n = int(input())
ss = [input() for _ in range(n)]

cnt = {}
for s in ss:
	for c in "abcdefghijklmnopqrstuvwxyz":
		t = s + c
		dt = deg(t)
		if dt in cnt:
			cnt[dt] += 1
		else:
			cnt[dt] = 1
			
ans = "-1"
for dt in cnt:
	if cnt[dt] == 1:
		t = "".join(itoc(i) * dt[i] for i in range(26))
		ans = t
		break
	
print(ans)