#world if 4>7
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==[]:
        A[i]=int(A[i])
        B[i]=int(B[i])
        if A[i]>B[i]:
            if A[i]!=4:
                Ou=A
            if B[i]!=7:
                Ou=A
        elif B[i]>A[i]:
            if A[i]!=7:
                Ou=B
            if B[i]!=4:
                Ou=B
        i+=1
for i in range(len(Ou)):
    Ou[i]=str(Ou[i])
    Ou=Ou.join("")
print(Ou)