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):
		S="".join(chr(j+97)*(H[j]+(i==j))for j in R(26))
		if S in D:D[S]+=1
		else:D[S]=1
for S in D:
	if D[S]<2:exit(print(S))
print(-1)