S = io.read()
T = {}
for i = 1, 26 do
    T[i] = false
end
for i = 1, #S do
    AtoI = S:sub(i, i):byte() - 96
    T[AtoI] = true
end

flag = true
for i = 1, 26 do    
    if not T[i] then
        print(string.char(96+i))
        flag = false
        break
    end
end
if flag then
    print("None")
end