A,B=raw_input().split()
Ac=list(A)
Bc=list(B)
for i in range(len(Ac)):
	if Ac[i]=='4':
		Ac[i]='7'
	elif Ac[i]=='7':
		Ac[i]='4'
for i in range(len(Bc)):
	if Bc[i]=='4':
		Bc[i]='7'
	elif Bc[i]=='7':
		Bc[i]='4'

if int(''.join(map(str,Ac))) > int(''.join(map(str,Bc))):
	print A
else:
	print B