#七対子

N = list(map(int,input().split()))
N.sort()


for i in range(13):
    if (N.count(N[i])==1):
        print(N[i])
    elif (N.count(N[i])!=2):
        pass
    elif (N.count(N[i])!=1)and(N.count(N[i])!=2):
        print("Impossible")