#!/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()