N = int(input())

dic = {}
for i in range(N):
    S = list(input())
    S.sort()
    nex = "".join(S)
    
    if nex in dic:
        dic[nex] += 1
    else:
        dic[nex] = 1

#print(dic)
abc = ["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"]

for ind,val in dic.items():
    if val == 1:
        nex2 = list(ind)
        for j in abc:
            nex3 = nex2 + [j]
            nex3.sort()
            nex4 = "".join(nex3)
            if nex4 not in dic:
                print(nex4)
                exit()

print(-1)