Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
import string
import itertools


def solve(s: str, K: int):

    part_set = set()
    for c in string.ascii_lowercase:
        for i in range(len(s)):
            if c == s[i]:
                part_set |= set(itertools.accumulate(s[i:]))
        if len(part_set) >= K:
            print(sorted(part_set)[K-1])
            return
                


# Generated by 1.1.4 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    s = next(tokens)  # type: str
    K = int(next(tokens))  # type: int
    solve(s, K)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
import string
import itertools


def solve(s: str, K: int):

    part_set = set()
    for c in string.ascii_lowercase:
        for i in range(len(s)):
            if c == s[i]:
                part_set |= set(itertools.accumulate(s[i:min(i+K,len(s))]))
        if len(part_set) >= K:
            print(sorted(part_set)[K-1])
            return
                


# Generated by 1.1.4 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    s = next(tokens)  # type: str
    K = int(next(tokens))  # type: int
    solve(s, K)

if __name__ == '__main__':
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#!/usr/bin/env python3
import sys
import string
import itertools


def solve(s: str, K: int):

    part_set = set()
    for c in string.ascii_lowercase:
        for i in range(len(s)):
            if c == s[i]:
                part_set |= set(itertools.accumulate(s[i:]))
        if len(part_set) >= K:
            print(sorted(part_set)[K-1])
            return
                


# Generated by 1.1.4 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    s = next(tokens)  # type: str
    K = int(next(tokens))  # type: int
    solve(s, K)

if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys
import string
import itertools


def solve(s: str, K: int):

    part_set = set()
    for c in string.ascii_lowercase:
        for i in range(len(s)):
            if c == s[i]:
                part_set |= set(itertools.accumulate(s[i:min(i+K,len(s))]))
        if len(part_set) >= K:
            print(sorted(part_set)[K-1])
            return
                


# Generated by 1.1.4 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    s = next(tokens)  # type: str
    K = int(next(tokens))  # type: int
    solve(s, K)

if __name__ == '__main__':
    main()