Buggy python submission (verdict: Runtime Error):
def main():
    s = input()
    k = int(input())
    ss = set()
    if len(s) <= 5:
        for i in range(len(s)):
            for j in range(i+1, len(s)+1):
                ss |= {s[i:j]}
    else:
        m = sorted(s)[0]
        for i in range(len(s)):
            if s[i] == m:
                for j in range(i+1, len(s)+1):
                    ss |= {s[i:j]}
    print(sorted(ss)[k-1])

if __name__ == "__main__":
    main()

Fixed submission:
s = input()
k = int(input())
n = len(s)
if n == 1:
    print(s)
elif n == 2:
    l2 = sorted(list(set([s[i:i+2] for i in range(n-1)])))
    print(l2[0])
elif n == 3:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    out = sorted(l1 + l2 + l3)
    print(out[k-1])
elif n == 4:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    out = sorted(l1 + l2 + l3 + l4)
    print(out[k-1])
else:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    l5 = list(set([s[i:i+5] for i in range(n-4)]))
    out = sorted(l1 + l2 + l3 + l4 + l5)
    print(out[k-1])

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    s = input()
    k = int(input())
    ss = set()
    for i in range(len(s)):
        for j in range(i+1, len(s)+1):
            ss |= {s[i:j]}
    print(sorted(ss)[k-1])

if __name__ == "__main__":
    main()

Fixed submission:
s = input()
k = int(input())
n = len(s)
if n == 1:
    print(s)
elif n == 2:
    l2 = sorted(list(set([s[i:i+2] for i in range(n-1)])))
    print(l2[0])
elif n == 3:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    out = sorted(l1 + l2 + l3)
    print(out[k-1])
elif n == 4:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    out = sorted(l1 + l2 + l3 + l4)
    print(out[k-1])
else:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    l5 = list(set([s[i:i+5] for i in range(n-4)]))
    out = sorted(l1 + l2 + l3 + l4 + l5)
    print(out[k-1])

Buggy python submission (verdict: Time Limit Exceeded):
def main():
    s = input()
    k = int(input())
    ss = []
    for i in range(len(s)):
        for j in range(i+1, len(s)+1):
            ss.append(s[i:j])
    print(sorted(set(ss))[k-1])

if __name__ == "__main__":
    main()

Fixed submission:
s = input()
k = int(input())
n = len(s)
if n == 1:
    print(s)
elif n == 2:
    l2 = sorted(list(set([s[i:i+2] for i in range(n-1)])))
    print(l2[0])
elif n == 3:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    out = sorted(l1 + l2 + l3)
    print(out[k-1])
elif n == 4:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    out = sorted(l1 + l2 + l3 + l4)
    print(out[k-1])
else:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    l5 = list(set([s[i:i+5] for i in range(n-4)]))
    out = sorted(l1 + l2 + l3 + l4 + l5)
    print(out[k-1])

Buggy python submission (verdict: Runtime Error):
s = input()
k = int(input())
n = len(s)
l2 = sorted([s[i:i+2] for i in range(n-1)])
l3 = sorted([s[i:i+3] for i in range(n-2)])
l4 = sorted([s[i:i+4] for i in range(n-3)])
l5 = sorted([s[i:i+5] for i in range(n-4)])
out = sorted([min(s),l2[0],l2[1],l2[2],l2[3],l3[0],l3[1],l3[2],l4[0],l4[1],l5[0]])
print(out[k-1])

Fixed submission:
s = input()
k = int(input())
n = len(s)
if n == 1:
    print(s)
elif n == 2:
    l2 = sorted(list(set([s[i:i+2] for i in range(n-1)])))
    print(l2[0])
elif n == 3:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    out = sorted(l1 + l2 + l3)
    print(out[k-1])
elif n == 4:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    out = sorted(l1 + l2 + l3 + l4)
    print(out[k-1])
else:
    l1 = list(set(s))
    l2 = list(set([s[i:i+2] for i in range(n-1)]))
    l3 = list(set([s[i:i+3] for i in range(n-2)]))
    l4 = list(set([s[i:i+4] for i in range(n-3)]))
    l5 = list(set([s[i:i+5] for i in range(n-4)]))
    out = sorted(l1 + l2 + l3 + l4 + l5)
    print(out[k-1])