local s = io.read()
local t = {}
for i = 1, 26 do
  t[i] = false
end
for i = 1, #s do
  local z = s:sub(i, i):byte() - 96
  t[z] = true
end
local f = true
for i = 1, 26 do
  if not t[i] then print(string.char(96 + i)) f = false break end
end
if f then print("None") end
