a.py

#!/usr/bin/python3

import sys

def f(s, k):
    o = [0] *  len(s)
    missing = k

    chars = list("abcdefghijklmnopqrstuvwxyz")

    for c in chars:
        for pos in reversed(range(0, len(s))):
            if c == s[pos] and (o[pos] == 0 or missing > -o[pos]):
                o[pos] = ord(c)
                missing = missing - 1
                if missing == 0:
                    return o

        right_empty = 0
        right_empty_new = 0

        for pos in reversed(range(0, len(s))):
            if o[pos] <= 0:
                o[pos] = -right_empty
                right_empty_new = right_empty_new + 1
            else:
                right_empty = right_empty_new

    return o

s = list(sys.argv[1])
k = int(sys.argv[2])
o = f(s, k)

# O(n)
for x in o:
    if x > 0:
        print(chr(x), end='')
print('')