n=input()

list1=[]
for i in range(13):
    if n.count(n[i])==2:
        i+=1
    elif n.count(n[i])==1:
        list1.append(n[i])
    elif n.count(n[i])>=3:
        list1.append(n[i])
if len(list1)==1:
    print(list1[0])
else:
    print("Impossible")