A,B=(list(i) for i in input().split())
if len(A)>len(B):
    Ou=A.join("")
elif len(B)>len(A):
    Ou=B.join("")
else:
    Ou=[]
    i=0
    while i<len(A) and Ou==[]:
#        print(A)
        A[i]=int(A[i])
        B[i]=int(B[i])
        if A[i]>B[i]:
            if A[i]!=7:
                Ou=A
            elif B[i]!=4:
                Ou=A
            else:
                Ou=B
        elif B[i]>A[i]:
            if A[i]!=4:
                Ou=B
            elif B[i]!=7:
                Ou=B
            else:
                Ou=A
        i+=1
for i in range(len(Ou)):
    Ou[i]=str(Ou[i])
    print(Ou)
if len(Ou)==1:
    an=Ou[0]
else:
    an="".join(Ou)
print(an)