N,M=map(int,raw_input().split())

n,m=N,M
nflag=True
while n!=0 and m!=0:
    a,b=n%10,m%10
    if (a,b) == (4,7):
        nflag=True
    elif (a,b) == (7,4):
        nflag=False
    elif a>b:
        nflag=True
    elif b>a:
        nflag=False
    n/=10
    m/=10
if n!=0:
    print N
elif m!=0:
    print M
elif nflag:
    print N
else:
    print M