Buggy python submission (verdict: Time Limit Exceeded):
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


# @lru_cache(size=None)
def slv(S, K):

    if len(S) <= 50:
        d = set([])
        for i in range(len(S)):
            for j in range(i + 1, len(S) + 1):
                d.add(S[i:j])

    # print('-----')

    import bisect
    S_ = [c for c in S]
    # S_.sort()
    idxs = []

    for k in sorted(set(S_)):
        # print('k', k)
        for i in range(len(S)):
            if S[i] == k:
                idxs.append(i)
        d = []

        for i in idxs:
            # print(i)
            for j in range(i + 1, len(S) + 1):
                t = "".join(S[i:j])
                if len(d) > K and d[-1] < t:
                    break
                if t not in d:
                    bisect.insort(d, t)
                    d = d[:K]
        if len(d) >= K:
            break
    print(d)
    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    S = [random.choice('abcdefghijklmnopqrstu') for _ in range(5000)]
    print(slv(S, K))


if __name__ == '__main__':
    main()

Fixed submission:
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


def slv(S, K):
    d = set([])
    for i in range(len(S)):
        for j in range(i + 1, i + 1 + K):
            d.add(S[i:j])

    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    # K = 3
    # S = ''.join([random.choice('abcdefghijklmnopqrstu') for _ in range(5000)])
    print(slv(S, K))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


# @lru_cache(size=None)
def slv(S, K):

    if len(S) <= 50:
        d = set([])
        for i in range(len(S)):
            for j in range(i + 1, len(S) + 1):
                d.add(S[i:j])

    # print('-----')

    import bisect
    S_ = [c for c in S]
    # S_.sort()
    idxs = []

    for k in sorted(set(S_)):
        # print('k', k)
        for i in range(len(S)):
            if S[i] == k:
                idxs.append(i)
        d = []

        for i in idxs:
            # print(i)
            for j in range(i + 1, len(S) + 1):
                if len(d) > K and d[-1] < S[i:j]:
                    break
                if S[i:j] not in d:
                    bisect.insort(d, S[i:j])
                    d = d[:K]
        if len(d) >= K:
            break
    # print(d)
    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    # S = [random.choice('abcdefghijklmnopqrstu') for _ in range(5000)]
    print(slv(S, K))


if __name__ == '__main__':
    main()

Fixed submission:
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


def slv(S, K):
    d = set([])
    for i in range(len(S)):
        for j in range(i + 1, i + 1 + K):
            d.add(S[i:j])

    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    # K = 3
    # S = ''.join([random.choice('abcdefghijklmnopqrstu') for _ in range(5000)])
    print(slv(S, K))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


# @lru_cache(size=None)
def slv(S, K):

    if len(S) <= 50:
        d = set([])
        for i in range(len(S)):
            for j in range(i + 1, len(S) + 1):
                d.add(S[i:j])

    # print('-----')

    import bisect
    S_ = [c for c in S]
    # S_.sort()
    idxs = []

    for k in sorted(set(S_)):
        # print('k', k)
        for i in range(len(S)):
            if S[i] == k:
                idxs.append(i)
        d = []

        for i in idxs:
            # print(i)
            for j in range(i + 1, len(S) + 1):
                if len(d) > K and d[-1] < S[i:j]:
                    break
                if S[i:j] not in d:
                    bisect.insort(d, S[i:j])
                    d = d[:K]
        if len(d) >= K:
            break
    # print(d)
    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    S = [random.choice('abcdefghijklmnopqrstu') for _ in range(5000)]
    print(slv(S, K))


if __name__ == '__main__':
    main()

Fixed submission:
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


def slv(S, K):
    d = set([])
    for i in range(len(S)):
        for j in range(i + 1, i + 1 + K):
            d.add(S[i:j])

    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    # K = 3
    # S = ''.join([random.choice('abcdefghijklmnopqrstu') for _ in range(5000)])
    print(slv(S, K))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


# @lru_cache(size=None)
def slv(S, K):

    if len(S) <= 50:
        d = set([])
        for i in range(len(S)):
            for j in range(i + 1, len(S) + 1):
                d.add(S[i:j])

    # print('-----')

    import bisect
    S_ = [c for c in S]
    # S_.sort()
    idxs = []

    for k in sorted(set(S_)):
        # print('k', k)
        for i in range(len(S)):
            if S[i] == k:
                idxs.append(i)
        d = []

        for i in idxs:
            for j in range(i + 1, len(S) + 1):
                if len(d) > K and d[-1] < S[i:j]:
                    break
                if S[i:j] not in d:
                    bisect.insort(d, S[i:j])
        if len(d) > K:
            break
    # print(d)
    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    print(slv(S, K))


if __name__ == '__main__':
    main()

Fixed submission:
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


def slv(S, K):
    d = set([])
    for i in range(len(S)):
        for j in range(i + 1, i + 1 + K):
            d.add(S[i:j])

    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    # K = 3
    # S = ''.join([random.choice('abcdefghijklmnopqrstu') for _ in range(5000)])
    print(slv(S, K))


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


# @lru_cache(size=None)
def slv(S, K):
    d = set([])
    for i in range(len(S)):
        for j in range(i + 1, len(S) + 1):
            d.add(S[i:j])
    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    print(slv(S, K))


if __name__ == '__main__':
    main()

Fixed submission:
import math
import random
import heapq
from collections import Counter, defaultdict
from decimal import Decimal, ROUND_HALF_UP, ROUND_CEILING
from functools import lru_cache, reduce
from itertools import combinations_with_replacement, product, combinations


def read_int():
    return int(input())


def read_int_n():
    return list(map(int, input().split()))


def read_float():
    return float(input())


def read_float_n():
    return list(map(float, input().split()))


def read_str():
    return input()


def read_str_n():
    return list(map(str, input().split()))


def slv(S, K):
    d = set([])
    for i in range(len(S)):
        for j in range(i + 1, i + 1 + K):
            d.add(S[i:j])

    return sorted(d)[K - 1]


def main():
    S = read_str()
    K = read_int()
    # K = 3
    # S = ''.join([random.choice('abcdefghijklmnopqrstu') for _ in range(5000)])
    print(slv(S, K))


if __name__ == '__main__':
    main()