from collections import defaultdict
n = int(input())
a = [[0] * 26 for i in range(n)]
t = defaultdict(int)
for i in range(n):
	s = input()
	for j in s:
		a[i][ord(j) - ord('a')] += 1

for i in range(n):
	for j in range(26):
		b = list(a[i])
		b[j] += 1
		t[tuple(b)] += 1

for x,y in t.items():
	if y == 1:
		g = []
		for i in range(26):
			for j in range(x[i]):
				g.append(chr(ord('a')+i))
		print(''.join(g))
		break
else:
	print("-1")