def larger(x, y):
	if len(x) > len(y): return x
	elif len(x) < len(y): return y
	else:
		for i in range(0, len(x)):
			if x[i] == '4' and y[i] == '7': return x
			elif x[i] == '7' and y[i] == '4': return y
			elif int(x[i]) > int(y[i]): return x
			elif int(x[i]) < int(y[i]): return y

A, B = raw_input().split()
print larger(A, B)