S=input()
dic={}
for i,alpha in enumerate(["a","b","c","d","e","f","g","h","i","j","k","l","m"]):
    dic[alpha]=i
    
b=[0,0,0,0,0,0,0,0,0,0,0,0,0]

for s in S:
    if(s in dic):
        b[dic[s]]+=1

count=[]
for i in range(3):
    count.append(b.count(i))
        
if(count[0]>1):
    print("Impossible")
    
elif(count[0]==1):
    if(count[1]==11 and count[2]==1):
        alpha=[k for k, v in dic.items() if v == b.index(0)]
        print(alpha[0])

elif(count[0]==0):
    for i in range(len(b)):
        alpha=[k for k, v in dic.items() if v == i]
        print(alpha[0])