print((function (f, g, s)
	local a,b = s:match("(%d+)%s(%d+)")
	return #a>#b and a or #b>#a and b or f(g,a,b)
end)(function (f, a, b)
	for i=1,#a do
		local c,d = f(a:sub(i,i),b:sub(i,i))
		if c~=d then return c>d and a or b end
	end
end, function (a, b)
	if (a=="4" and b=="7") or (a=="7" and b=="4") then return b,a end
	return a,b
end, io.stdin:read("*l")))