A = {i:0 for i in "abcdefghijklm"}
for i in input():
    if i in A:
        A[i]+=1
        
if list(A.values()).count(0)>1:
    print("Impossible")
elif max(A.values()) > 1:
    for i,j in A.items():
        if j == 0:
            print(i)
else:
    for i in sorted(A):
        print(i)