while True:
    n = int(raw_input())
    if n == 0: break

    names = []
    for i in range(n):
        name = raw_input()
        while name[-1] < 'A' or name[-1] > 'Z':
            name = name[:-1]
        names.append(name)
    names.sort()

    a, b = names[n / 2 - 1], names[n / 2]
    ans = a
    for i in range(len(a) - 1):
        if a[i] != 'Z':
            z = a[:i] + chr(ord(a[i]) + 1)
            if z < b:
                ans = z
                break

    print ans