a.py

#!/usr/bin/python3

import sys

s = list(sys.argv[1])
k = int(sys.argv[2])
r = 0

o = [''] *  len(s)

chars = list("abcdefghijklmnopqrstuvwxyz")

# constant, up to 26
for c in chars:
    # O(n)
    pos = 0
    while pos < len(s):
        if s[pos] == c:
            o[pos] = c
            r = r + 1
            if r == k:
                break
        pos = pos + 1
    if r == k:
        break

# O(n)
for x in o:
    if x != '':
        print(x, end='')
print('')