Buggy python submission (verdict: Runtime Error):
mojiretsu=input()
kakunin="abcdefghijklmnopqrstuvwxyz"
nagasa=len(mojiretsu)
bunkai=[]
for i in range(nagasa): 
    bunkai.append(mojiretsu[i:i+1])
bunkai1=list(set(bunkai))
bunkai1.sort()
ans="None"
NGFlug=False
j=0

for k in range(len(bunkai)):
    if bunkai1[k]!=kakunin[k:k+1]:
        ans=kakunin[k:k+1]
        NGFlug=True
        break

    j+=1

if NGFlug==True:
    
    print(ans)
elif j<27:
    print(kakunin[j:j+1])
else:
    print(ans)

Fixed submission:
S=input()

if "a" not in S:
    print("a")
elif "b"  not in S:
    print("b")
elif "c"  not in S:
    print("c")
elif "d"  not in S:
    print("d")
elif "e"  not in S:
    print("e")
elif "f"  not in S:
    print("f")
elif "g"  not in S:
    print("g")
elif "h"  not in S:
    print("h")
elif "i"  not in S:
    print("i")
elif "j"  not in S:
    print("j")
elif "k"  not in S:
    print("k")
elif "l"  not in S:
    print("l")
elif "m"  not in S:
    print("m")
elif "n"  not in S:
    print("n")
elif "o"  not in S:
    print("o")
elif "p"  not in S:
    print("p")
elif "q"  not in S:
    print("q")
elif "r"  not in S:
    print("r")
elif "s"  not in S:
    print("s")
elif "t"  not in S:
    print("t")
elif "u"  not in S:
    print("u")
elif "v"  not in S:
    print("v")
elif "w"  not in S:
    print("w")
elif "x"  not in S:
    print("x")
elif "y"  not in S:
    print("y")
elif "z"  not in S:
    print("z")
else:
    print("None")

Buggy python submission (verdict: Runtime Error):
mojiretsu=input()
kakunin="abcdefghijklmnopqrstuvwxyz"
nagasa=len(mojiretsu)
bunkai=[]
for i in range(nagasa): 
    bunkai.append(mojiretsu[i:i+1])
bunkai1=list(set(bunkai))
bunkai1.sort()
ans="None"
NGFlug=False
j=1

for k in range(len(bunkai)):
    if bunkai1[k]!=kakunin[k:k+1]:
        ans=kakunin[k:k+1]
        NGFlug=True
        break

    j+=1

if NGFlug==True:
    
    print(ans)
elif j<27:
    print(kakunin[j:j+1])
else:
    print(ans)

Fixed submission:
S=input()

if "a" not in S:
    print("a")
elif "b"  not in S:
    print("b")
elif "c"  not in S:
    print("c")
elif "d"  not in S:
    print("d")
elif "e"  not in S:
    print("e")
elif "f"  not in S:
    print("f")
elif "g"  not in S:
    print("g")
elif "h"  not in S:
    print("h")
elif "i"  not in S:
    print("i")
elif "j"  not in S:
    print("j")
elif "k"  not in S:
    print("k")
elif "l"  not in S:
    print("l")
elif "m"  not in S:
    print("m")
elif "n"  not in S:
    print("n")
elif "o"  not in S:
    print("o")
elif "p"  not in S:
    print("p")
elif "q"  not in S:
    print("q")
elif "r"  not in S:
    print("r")
elif "s"  not in S:
    print("s")
elif "t"  not in S:
    print("t")
elif "u"  not in S:
    print("u")
elif "v"  not in S:
    print("v")
elif "w"  not in S:
    print("w")
elif "x"  not in S:
    print("x")
elif "y"  not in S:
    print("y")
elif "z"  not in S:
    print("z")
else:
    print("None")

Buggy python submission (verdict: Runtime Error):
mojiretsu=input()
kakunin="abcdefghijklmnopqrstuvwxyz"
nagasa=len(mojiretsu)
bunkai=[]
for i in range(nagasa): 
    bunkai.append(mojiretsu[i:i+1])
bunkai1=list(set(bunkai))
bunkai1.sort()
ans="None"


for k in range(len(bunkai)):
    if bunkai1[k]!=kakunin[k:k+1]:
        ans=kakunin[k:k+1]
        
        break
    
print(ans)

Fixed submission:
S=input()

if "a" not in S:
    print("a")
elif "b"  not in S:
    print("b")
elif "c"  not in S:
    print("c")
elif "d"  not in S:
    print("d")
elif "e"  not in S:
    print("e")
elif "f"  not in S:
    print("f")
elif "g"  not in S:
    print("g")
elif "h"  not in S:
    print("h")
elif "i"  not in S:
    print("i")
elif "j"  not in S:
    print("j")
elif "k"  not in S:
    print("k")
elif "l"  not in S:
    print("l")
elif "m"  not in S:
    print("m")
elif "n"  not in S:
    print("n")
elif "o"  not in S:
    print("o")
elif "p"  not in S:
    print("p")
elif "q"  not in S:
    print("q")
elif "r"  not in S:
    print("r")
elif "s"  not in S:
    print("s")
elif "t"  not in S:
    print("t")
elif "u"  not in S:
    print("u")
elif "v"  not in S:
    print("v")
elif "w"  not in S:
    print("w")
elif "x"  not in S:
    print("x")
elif "y"  not in S:
    print("y")
elif "z"  not in S:
    print("z")
else:
    print("None")