n=int(input())
s=[input() for _ in range(n)]
t=[[0 for _ in range(26)]for _ in range(n)]
for i in range(n):
	for j in range(26):
		t[i][j]=s[i].count(chr(ord('a')+j))
for i in range(n):
	T=True
	for j in range(n):
		if i!=j:
			L=False
			for k in range(26):
				if t[i][k]<t[j][k]:
					L=True
					break 
			if not L:
				T=False
	if T:
		print(s[i]+'a')
		exit()
print(-1)