local S = io.read("*l")

local abc = "abcdefghijklmnopqrstuvwxyz"
local have_abc_s = {}
for i = 1, abc:len()  do
	have_abc_s[abc:byte(i)] = false
end

for i = 1, S:len() do
	have_abc_s[S:byte(i)] = true
end

local out = "None"
for i = 1, abc:len() do
	if not have_abc_s[abc:byte(i)] then
		out = abc:sub(i, i)
		break
	end
end

print(out)
