import sys
A,B=input().split()

if len(A)<len(B):
    print(B)
    sys.exit()
elif len(A)>len(B):
    print(A)
    sys.exit()

f=0
n=len(A)
for i in range(n):
    if f!=0:
        continue
    
    T=(A[i],B[i])
    if T==("4","7") or T==("7","4"):
        if A[i]=="4":
            f=1
        else:
            f=-1
    elif A[i]!=B[i]:
        if int(A[i])<int(B[i]):
            f=-1
        else:
            f=1

if f==1:
    print(A)
else:
    print(B)