Pyramidic

n = int(input())
d = {}
best_c = ''
best_x = 0


def track_best(c):
    global best_c, best_x
    x = d[c]
    if x > best_x:
        best_c = c
        best_x = x


def verify(line):
    keys = list(d.keys())[:]
    for c in keys:
        x = d[c] + 2
        while x > 0:
            if line.find(c * x) < 0:
                x -= 2
                if x < 0:
                    del d[c]
            else:
                d[c] = x
                track_best(c)
                break

    for c in line:
        if c not in d:
            d[c] = 1
            track_best(c)


for i in range(n):
    line = input()

    if i == 0:
        for c in line:
            if c not in d:
                d[c] = 1
                track_best(c)
    else:
        verify(line)

[print(best_c * i) for i in range(1, best_x + 1, 2)]