def highestValuePalindrome(s, n, k):
    s = list(s)  
    left = 0
    right = n - 1
    mismatches = set()
    while left < right:
        if s[left] != s[right]:
            s[left] = s[right] = max(s[left], s[right])
            mismatches.add(left)
            k -= 1
        if k < 0: 
            return '-1'
        left += 1
        right -= 1
        
    left = 0
    right = n - 1
    while left <= right:
        if left == right: 
            if k > 0:
                s[left] = '9'
        elif s[left] != '9':  
            if left in mismatches: 
                if k >= 1:
                    s[left] = s[right] = '9'
                    k -= 1
            elif k >= 2:  
                s[left] = s[right] = '9'
                k -= 2
        left += 1
        right -= 1

    return ''.join(s)

    if n == 1 and k > 0:
        return '9'
    count = sum([1 for i in range(n//2) if s[i] != s[-i-1]])
    if k < count:
        return '-1'
    ex = k-count
    s = list(s)
    for i in range(n//2):
        if s[i] == s[-i-1] != '9' and ex > 1:
            s[i] = s[-i-1] = '9'
            ex -=2
        elif s[i] != s[-i-1]:
            if s[i] == '9' or s[-i-1] == '9':
                s[i] = s[-i-1] = '9'
            else:
                if ex > 0:
                    s[i] = s[-i - 1] = '9'
                    ex -= 1
                else:
                    s[i] = s[-i - 1] = max(s[i], s[-i - 1])
    if ex > 0 and n % 2 == 1:
        s[n//2] = '9'
    return ''.join(s)

#!/bin/python3

import math
import os
import random
import re
import sys

#
# Complete the 'highestValuePalindrome' function below.
#
# The function is expected to return a STRING.
# The function accepts following parameters:
#  1. STRING s
#  2. INTEGER n
#  3. INTEGER k
#
_marks=[]
def _create_palindrom(s):
    count=0
    s=list(s)
    for i in range(len(s)//2):
        #check string
        if (s[i]<s[len(s)-i-1]):
            count+=1
            s[i]=s[len(s)-i-1]
            _marks.append(i)
        elif s[i]>s[len(s)-i-1]:
            s[len(s)-i-1]=s[i]
            count+=1
            _marks.append(i)
    return s, count
            
    
def _maximize_palindrome(s, k, count):
   if k>0 and len(s)==1:
       s[0]='9'
       return
   for i in range(len(s)//2):
       #change the current symbol
       if k>1 and s[i]<'9' and i not in _marks:
           s[i]='9'
           s[len(s)-i-1]='9'
           k-=2
       elif count>0 and s[i]<'9' and i in _marks:    
            s[i]='9'
            s[len(s)-i-1]='9'
            if count>1:
               count-=2
            else:
                count-=1
                k-=1
   #change the middle         
   if k==1 and len(s)%2>0 and count>=0:
         s[len(s)//2]='9'
           
           

def highestValuePalindrome(s, n, k):
    res, count=_create_palindrom(s)
    if count>k:
        return '-1'
    if count<k:
        _maximize_palindrome(res, k-count, count) 
    return ''.join(res)
    
    # Write your code here

if __name__ == '__main__':
    fptr = open(os.environ['OUTPUT_PATH'], 'w')

    first_multiple_input = input().rstrip().split()

    n = int(first_multiple_input[0])

    k = int(first_multiple_input[1])

    s = input()

    result = highestValuePalindrome(s, n, k)

    fptr.write(result + '\n')

    fptr.close()

"""
The code fails for long strings probably because the environment times out but I ran locally and it's working as expected
"""

def highestValuePalindrome(s, n, k): 
    # Write your code here
    count = 0
    sarr = [c for c in s]
    for i in range(n//2):
        if sarr[i] == sarr[n-1-i] and sarr[i] != '9' and k>1:            
                sarr[i] = '9'
                sarr[n-i-1] = '9'
                k-=2
        elif sarr[i] == sarr[n-1-i] and k == 1:
            continue
        elif sarr[i] != sarr[n-1-i] and k > 1:
            if sarr[i] == '9':
                sarr[n-1-i] = '9'
                k -= 1
            elif sarr[n-1-i] == '9':
                sarr[i] = '9'
                k -= 1
            else:
                sarr[i] = '9'
                sarr[n-i-1] = '9'
                k-=2
            
        elif sarr[i] != sarr[n-1-i] and k == 1:
            if sarr[i] > sarr[n-1-i]:
                sarr[n-1-i] = sarr[i]
            else:
                sarr[i] = sarr[n-1-i]
            k -= 1
        if k == 0:
            break
    if k == 1:
        if sarr[n//2] != '9':
            sarr[n//2] = '9'
            k -= 1
    print(sarr)
    retval = 0
    for i in range(n//2):
        if sarr[i] != sarr[n-1-i]:
            retval = -1
            print("will return -1")
    if retval <0:
        return str(retval)
    else:
        return ''.join(sarr)