S = sorted(list(set(list(input()))))
alfabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
count = 0
for char in alfabet:
    if not(char in S):
        print(char)
        break
    count+=1
if count == 26:
    print("None")
048b79cdfb664c998b14495fa700993d884c0c65
<mask token>

<mask token>
for char in alfabet:
    if not char in S:
        print(char)
        break
    count += 1
if count == 26:
    print('None')

S = sorted(list(set(list(input()))))
alfabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
    'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']
count = 0
for char in alfabet:
    if not char in S:
        print(char)
        break
    count += 1
if count == 26:
    print('None')

S = sorted(list(set(list(input()))))
alfabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
count = 0
for char in alfabet:
    if not(char in S):
        print(char)
        break
    count+=1
if count == 26:
    print("None")