N = input()
l = list(N)
N1 = "abcdefghijklmabcdefghijklm"
l1 = list(N1)
flg = 0

for i in l:
    try:
        l1.remove(i)
    except:
        print("Impossible")
        flg = 1
        break

if flg == 0:
    if l1[0] == "a":
        if l1[1] == "a":
            print("a")
        else:
            for i in l1:
                print(i)
    else:
        print(l1[0])