n = int(input())
s = []
for i in range(n):
	s.append(''.join(sorted(input())))
s.sort()
d = {}
for si in s:
	if si in d:
		d[si] = False
	else:
		d[si] = True
s = []
for key in d:
	if d[key]:
		s.append(key)

if n == 1:
	print(s[0] + "a")
else:
	f = True
	for i in range(n-1):
		a, b = s[i], s[i+1]
		if a == b: continue
		if len(a) < len(b):
			print(a + "a")
		else:
			for j in range(len(a)):
				if a[j] != b[j]:
					for c in "abcdefghijklmnopqrstuvwxyz":
						if c != a[j] and c != b[j]:
							print(a+c)
							break
					break
		f = False
		break
	if f: print(-1)