from lib import *

data = open(0).read().strip()
second = True

p = ""
for row in zip(*data.splitlines()):
    counts = {}
    for l in LETTERS_LOWER:
        counts[l] = row.count(l)
    maxc = 0
    minc = 999
    c = ""
    for l, count in counts.items():
        if second:
            if count > 0 and count < minc:
                minc = count
                c = l
        else:
            if count > maxc:
                maxc = count
                c = l
    p += c

print(p)