import sys

tmp_s = "abcdefghijklm"
dic = {}

for i in tmp_s:
    dic[i] = 0

s = raw_input()

n0 = len(tmp_s)
n1 = 0
n2 = 0

for i in s:
    if i not in tmp_s:
        print "Impossible"
        sys.exit()
    elif dic[i] == 3:
        print "Impossible"
        sys.exit()
    else:
        dic[i] += 1
        if dic[i] == 2:
            n1 -= 1
            n2 += 1
        elif dic[i] == 1:
            n0 -= 1
            n1 += 1

keys = dic.keys()

if n2 == 1 and n0 == 1:
    for i in keys:
        if dic[i] == 0:
            print i
            break
elif n0 == 0 and n2 == 0:
    for i in tmp_s:
        print i
else:
    print "Impossible"