N = int(input())
L = [[0]*26 for _ in range(N)]
cand = []
for i in range(N):
    s = input()
    cand.append(s)
    for si in s:
        j = ord(si)-ord("a")
        L[i][j] += 1

for i in range(N):
    OK = [False] * N
    OK[i] = True
    skips = set()
    for j in range(N):
        if i==j:continue
        flag = False
        for k in range(26):
            if L[j][k] > L[i][k]:
                flag = True
                if L[j][k] - L[i][k] == 1:
                    skips.add(k)
        OK[j] = True
    if all(OK) and len(skips) < 26: break

if not all(OK):
    print("-1")
else:
    if len(skips) == 26:
        print("-1")
    else:
        for x in range(26):
            if not x in skips:
                break
        ans = cand[i] + chr(x+ord("a"))
        print(ans)