I,R=input,range
D={}
for n in R(int(I())):
	H=[0]*26
	for s in I():H[ord(s)-97]+=1
	for i in R(26):
		h=sum((H[j]+(i==j))*11**j for j in R(26))
		if h in D:D[h]+=1
		else:D[h]=1
for h in D:
	if D[h]<2:
		a=""
		for i in R(26):
			for n in R(h//(11**i)%11):a+=chr(i+97)
		exit(print(a))
print(-1)