s=input()
s_set=set(s)
array=[]
count=0
for i in s_set:
    if s.count(i)==1:
        array.append(i)
    elif s.count(i)==2:
        count+=1
    else:
        continue
if len(s_set)==7 and count==6:
    print(array[0])
else:
    print("Impossible")