from collections import defaultdict

n = int(input())
S = [list(input()) for _ in range(n)]
alpha = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

d = defaultdict(int)

for i in range(n):
    for j in range(26):
        tmp = S[i]+[alpha[j]]
        tmp.sort()
        d["".join(tmp)] += 1
        
for key in d.keys():
    if d[key] == 1:
        print(key)
        exit()
        
print(-1)