local a, b = io.read("*n", "*n")
local ca, cb = a, b
local adigs, bdigs = {}, {}
while(0 < ca) do
  table.insert(adigs, ca % 10)
  ca = math.floor(ca / 10)
end
while(0 < cb) do
  table.insert(bdigs, cb % 10)
  cb = math.floor(cb / 10)
end
if(a == 0) then print(b)
elseif(b == 0) then print(a)
elseif(#adigs < #bdigs) then print(b)
elseif(#bdigs < #adigs) then print(a)
else
  for i = #adigs, 1, -1 do
    if(adigs[i] == 4) then
      if(bdigs[i] == 7) then print(a) break end
    elseif(adigs[i] == 7) then
      if(bdigs[i] == 4) then print(b) break end
    end
    if(adigs[i] < bdigs[i]) then print(b) break
    elseif(bdigs[i] < adigs[i]) then print(a) break
    end
  end
end