def find_string(strings):
    char_count = {}
    for string in strings:
        for char in string:
            char_count[char] = char_count.get(char, 0) + 1
    unique_char = None
    for char, count in char_count.items():
        if count == 1:
            unique_char = char
            break
    if unique_char is None:
        return -1
    for string in strings:
        if unique_char in string:
            string += 'a'
            return ''.join(sorted(string))
N = int(input())
strings = []
for _ in range(N):
    strings.append(input().strip())
result = find_string(strings)
print(result)