# import bisect

# import copy

# import fractions

# import math

# import numpy as np

# from collections import Counter, deque

# from itertools import accumulate,permutations, combinations,combinations_with_replacement,product



def resolve():

    s=list(eval(input()))



    # アルファベット小文字→(97, 123)

    # アルファベット大文字→(65, 91)

    # 半角数字→(48, 58)

    # ひらがな→(12353, 12436)

    # カタカナ→(12449, 12532+1)

    # 全角数字→(65296, 65306)



    def moji_list(*args):

        moji = []

        for i in range(len(args)):

            moji.extend([chr(j) for j in range(args[i][0], args[i][1])])

        return moji



    lowercase = moji_list((97,123))



    for i in s:

        if i in lowercase:

            lowercase.remove(i)

    print((lowercase[0] if len(lowercase)!=0 else 'None'))



resolve()
# import bisect

# import copy

# import fractions

# import math

# import numpy as np

# from collections import Counter, deque

# from itertools import accumulate,permutations, combinations,combinations_with_replacement,product



def resolve():

    s=list(eval(input()))



# アルファベット小文字→(97, 123)を入力に入れる

# アルファベット大文字→(65, 91)

# 半角数字→(48, 58)

# ひらがな→(12353, 12436)

# カタカナ→(12449, 12532+1)

# 全角数字→(65296, 65306)







    def moji_list(*args):

        moji = []

        for i in range(len(args)):

            moji.extend([chr(j) for j in range(args[i][0], args[i][1])])

        return moji



    lowercase = moji_list((97,123))



    print((min(set(lowercase)-set(s) or ['None'])))



resolve()
# import bisect
# import copy
# import fractions
# import math
# import numpy as np
# from collections import Counter, deque
# from itertools import accumulate,permutations, combinations,combinations_with_replacement,product
def resolve():
    s = list(eval(input()))
    # アルファベット小文字→(97, 123)
    # アルファベット大文字→(65, 91)
    # 半角数字→(48, 58)
    # ひらがな→(12353, 12436)
    # カタカナ→(12449, 12532+1)
    # 全角数字→(65296, 65306)
    def moji_list(*args):
        moji = []
        for i in range(len(args)):
            moji.extend([chr(j) for j in range(args[i][0], args[i][1])])
        return moji

    lowercase = moji_list((97, 123))
    for i in s:
        if i in lowercase:
            lowercase.remove(i)
    print((lowercase[0] if len(lowercase) != 0 else "None"))


resolve()

# import bisect
# import copy
# import fractions
# import math
# import numpy as np
# from collections import Counter, deque
# from itertools import accumulate,permutations, combinations,combinations_with_replacement,product
def resolve():
    s = list(eval(input()))
    # アルファベット小文字→(97, 123)を入力に入れる
    # アルファベット大文字→(65, 91)
    # 半角数字→(48, 58)
    # ひらがな→(12353, 12436)
    # カタカナ→(12449, 12532+1)
    # 全角数字→(65296, 65306)
    def moji_list(*args):
        moji = []
        for i in range(len(args)):
            moji.extend([chr(j) for j in range(args[i][0], args[i][1])])
        return moji

    lowercase = moji_list((97, 123))
    print((min(set(lowercase) - set(s) or ["None"])))


resolve()

-    # アルファベット小文字→(97, 123) +    # アルファベット小文字→(97, 123)を入力に入れる -    for i in s: -        if i in lowercase: -            lowercase.remove(i) -    print((lowercase[0] if len(lowercase) != 0 else "None")) +    print((min(set(lowercase) - set(s) or ["None"])))