#include <bits/stdc++.h>
using namespace std;
string richieRich(string s, int n, int k){
 int lives=k;
 vector<bool> mod(n,false); 
 string temp(s); 
 
 for (int i=0;i<n/2;i++) 
 {
 if (temp[i]!=temp[n-i-1]) {mod[i]=true;lives--;}
 if (temp[i]<temp[n-i-1]) temp[i]=temp[n-i-1]; else if (temp[i]>temp[n-i-1]) temp[n-i-1]=temp[i];
 if (lives<0) return "-1"; 
 }
 int j=0;
 while ((lives>0)&&(j<n/2))
 {
 if (temp[j]!='9'){
 if (mod[j]) lives++;
 if (lives>1) {temp[j]='9';temp[n-j-1]='9'; lives-=2;}
 } 
 j++; 
 }
 if (n%2==1) {if (lives>0) temp[n/2]='9';} 
 return temp;
}
int main() {
 int n;
 cin >> n;
 int k;
 cin >> k;
 string s;
 cin >> s;
 string result = richieRich(s, n, k);
 cout << result << endl;
 return 0;
}