Buggy python submission (verdict: Runtime Error):
import sys

s = sys.stdin.readline().rstrip()
k = int(sys.stdin.readline().rstrip())
n = len(s)

def main():
    res = set()
    for i in range(n):
        for j in range(i, i+4):
            res.add(s[i:j+1])
    
    return sorted(res)[k-1]

if __name__ == '__main__':
    ans = main()
    print(ans)

Fixed submission:
#                         author:  kagemeka 
#                         created: 2019-11-07 13:42:36(JST)
import sys
# import collections
# import math
# import string
# import bisect
# import re
# import itertools
# import statistics
# import functools
# import operator

def main():
    s = sys.stdin.readline().rstrip()
    k = int(sys.stdin.readline().rstrip())
    n = len(s)
    
    substrings = []
    for i in range(1, k+1):
        for j in range(n-i+1):
            substring = s[j:j+i]
            substrings.append(substring)
    substrings = sorted(list(set(substrings)))
    print(substrings[k-1])


if __name__ == "__main__":
    # execute only if run as a script
    main()

Buggy python submission (verdict: Time Limit Exceeded):
import sys

s = sys.stdin.readline().rstrip()
k = int(sys.stdin.readline().rstrip())
n = len(s)

def main():
    res = set()
    for i in range(n):
        for j in range(i, n):
            res.add(s[i:j+1])
    return sorted(res)[k-1]

if __name__ == '__main__':
    ans = main()
    print(ans)

Fixed submission:
#                         author:  kagemeka 
#                         created: 2019-11-07 13:42:36(JST)
import sys
# import collections
# import math
# import string
# import bisect
# import re
# import itertools
# import statistics
# import functools
# import operator

def main():
    s = sys.stdin.readline().rstrip()
    k = int(sys.stdin.readline().rstrip())
    n = len(s)
    
    substrings = []
    for i in range(1, k+1):
        for j in range(n-i+1):
            substring = s[j:j+i]
            substrings.append(substring)
    substrings = sorted(list(set(substrings)))
    print(substrings[k-1])


if __name__ == "__main__":
    # execute only if run as a script
    main()

Buggy python submission (verdict: Time Limit Exceeded):
#                         author:  kagemeka 
#                         created: 2019-11-07 13:42:36(JST)
import sys
# import collections
# import math
# import string
# import bisect
# import re
# import itertools
# import statistics
# import functools
# import operator

def main():
    s = sys.stdin.readline().rstrip()
    k = int(sys.stdin.readline().rstrip())
    n = len(s)

    substrings = []
    for i in range(1, n+1):
        for j in range(n-i+1):
            substring = s[j:j+i]
            substrings.append(substring)
            
    substrings = sorted(list(set(substrings)))
    print(substrings[k-1])


if __name__ == "__main__":
    # execute only if run as a script
    main()


Fixed submission:
#                         author:  kagemeka 
#                         created: 2019-11-07 13:42:36(JST)
import sys
# import collections
# import math
# import string
# import bisect
# import re
# import itertools
# import statistics
# import functools
# import operator

def main():
    s = sys.stdin.readline().rstrip()
    k = int(sys.stdin.readline().rstrip())
    n = len(s)
    
    substrings = []
    for i in range(1, k+1):
        for j in range(n-i+1):
            substring = s[j:j+i]
            substrings.append(substring)
    substrings = sorted(list(set(substrings)))
    print(substrings[k-1])


if __name__ == "__main__":
    # execute only if run as a script
    main()