Buggy python submission (verdict: Runtime Error):
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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']
if s3 == s4:
    print('None')
else:
    for i in range(int(len(s4))):
        if s3[i] != s4[i]:
            print(s4[i])
            break

Fixed submission:
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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']
if s3 == s4:
    print('None')
else:
    for i in range(int(len(s3))):
        if s3[i] != s4[i]:
            print(s4[i])
            break
        if i == len(s3)-1:
            print(s4[i+1])

Buggy python submission (verdict: Runtime Error):
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s2 = list(set(s2))
s2.sort()
s3 = ['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']
if s2 == s3:
    print('None')
else:
    for i in range(int(len(s3))):
        if s2[i] != s3[i]:
            print(s3[i])
            break

Fixed submission:
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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']
if s3 == s4:
    print('None')
else:
    for i in range(int(len(s3))):
        if s3[i] != s4[i]:
            print(s4[i])
            break
        if i == len(s3)-1:
            print(s4[i+1])

Buggy python submission (verdict: Runtime Error):
s = input()
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s2 = list(set(s2))
s2.sort()
s3 = ['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']
if s2 == s3:
    print('None')
else:
    for i in range(int(len(s3))):
        if s2[i] != s3[i]:
            print(s3[i])
            break

Fixed submission:
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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']
if s3 == s4:
    print('None')
else:
    for i in range(int(len(s3))):
        if s3[i] != s4[i]:
            print(s4[i])
            break
        if i == len(s3)-1:
            print(s4[i+1])

Buggy python submission (verdict: Runtime Error):
s = input()
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s2 = list(set(s2))
s2.sort()
s3 = ['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']
if s2 == s3:
    print('None')
else:
    for i in range(len(s3)):
        if s2[i] != s3[i]:
            print(s3[i])
            break

Fixed submission:
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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']
if s3 == s4:
    print('None')
else:
    for i in range(int(len(s3))):
        if s3[i] != s4[i]:
            print(s4[i])
            break
        if i == len(s3)-1:
            print(s4[i+1])

Buggy python submission (verdict: Runtime Error):
s = input()
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s2 = list(set(s2))
s2.sort()
s3 = ['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']
if s2 == s3:
    print('None')
else:
    for i in range(len(s)):
        if s2[i] != s3[i]:
            print(s3[i])
            break

Fixed submission:
s = list(input())
s2 = []
for i in range(len(s)):
    s2.append(s[i])
s3 = list(set(s2))
s3.sort()
s4 = ['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']
if s3 == s4:
    print('None')
else:
    for i in range(int(len(s3))):
        if s3[i] != s4[i]:
            print(s4[i])
            break
        if i == len(s3)-1:
            print(s4[i+1])