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