from functools import cmp_to_key

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y = x.split('\n')
z = ''.join(set(''.join(y)))
y1 = [a[0] + a[1] for a in y if a]
y2 = [a[1] + a[2] for a in y if a]
y = set(y1 + y2)


def comparator(m, n):
    if m + n in y:
        return -1
    if m == n:
        return 0
    return 1

print(''.join(sorted(z, key=cmp_to_key(comparator))))

