S=input()
list1=["a","b","c","d","e","f","g","h","i","j","k","l","m"]
list2=["a","b","c","d","e","f","g","h","i","j","k","l","m"]
for i in range(13):
    if S[i] in list1:
        list1.remove(S[i])
#print(list1)
if len(list1)==1:
    print(list1[0])
elif len(list1)==0:
    for j in range(13):
        print(list2[j])
else:
    print("Impossible")