Problem: Marina loves strings of the same length and Vasya loves when there is a third string, different from them in exactly t characters. Help Vasya find at least one such string.

More formally, you are given two strings s1, s2 of length n and number t. Let's denote as f(a, b) the number of characters in which strings a and b are different. Then your task will be to find any string s3 of length n, such that f(s1, s3) = f(s2, s3) = t. If there is no such string, print  - 1.

Input

The first line contains two integers n and t (1 ≤ n ≤ 105, 0 ≤ t ≤ n).

The second line contains string s1 of length n, consisting of lowercase English letters.

The third line contain string s2 of length n, consisting of lowercase English letters.

Output

Print a string of length n, differing from string s1 and from s2 in exactly t characters. Your string should consist only from lowercase English letters. If such string doesn't exist, print -1.

Examples

Input

3 2
abc
xyc


Output

ayd

Input

1 0
c
b


Output

-1

Solution (in JAVA):

Solution:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Vector;

/**
 * Created by nursultan on 10/10/15.
 */
public class Test99 {

    static void solve(String a, String b, int n, int t){

        Vector<Integer> same = new Vector<>();
        Vector<Integer> diff = new Vector<>();

        // adding indices of chars that are = and !=
        for(int i = 0; i < n; i++){
            if(a.charAt(i) == b.charAt(i)){
                same.add(i);
            } else {
                diff.add(i);
            }
        }

        char[] out = new char[n];

        for(int i = 0; i < n; i++){
            out[i] = another(a.charAt(i), b.charAt(i));
//            System.out.print(out[i]);
        }
//

        // t - kolichestvo simvolov v kotorih est' razli4ie, n - kolvo simvolov
        while(t < n){
            if(same.size() >= 1){
                final int i = same.lastElement();
                int last = same.size() - 1;
                same.removeElementAt(last);
                out[i] = a.charAt(i);
                t++;
            } else if(diff.size() >= 2){
                int last = diff.size() - 1;
                final int i = diff.lastElement();
                diff.removeElementAt(last);
                final int j = diff.lastElement();
                diff.removeElementAt(last - 1);
                out[i] = a.charAt(i);
                out[j] = b.charAt(j);
                t++;
            } else {
                System.out.println("-1");
                return;
            }
        }

        for(int i = 0; i < n; i++){
            System.out.print(out[i]);
        }
    }


    static char another(char x, char y){
        // a!=c && b!=c
        for(char r = 'a'; r <= 'z'; r++){
            if(x!=r && y!=r){
                return r;
            }
        }
        return 'z';
    }


    public static void main(String[] args) throws IOException{

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        String digits = br.readLine();

        int n = Integer.parseInt(digits.split(" ")[0]);
        int t = Integer.parseInt(digits.split(" ")[1]);

        String s1 = br.readLine();
        String s2 = br.readLine();

        solve(s1, s2, n, t);

    }

}