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.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;


public class CF_324_C {
    private static char getR(char a, char b){
        int x1 = Math.max(a, b);
        int x2 = Math.min(a, b);
        char ans;
        if(x1 == x2+1){
            if(x1 == 'z')return 'a';
            if(x2 == 'a')return 'z';
            ans = (char)(x2-1);
        }else{
            ans = (char)(x2+1);
        }
        return ans;
    }
    
    private static char getR2(char a, char b){
        int x = a;
        while( x == a ||  x == b){
            x = 'a' + (int)(Math.random() * (('z' - 'a') + 1));
        }
        return (char)x;
    }
    
    public static void main(String[] args) throws IOException{
        PrintWriter pw = new PrintWriter(System.out, true);
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        int n, t;
    
        String[] line = input.readLine().split(" ");
        n = Integer.parseInt(line[0]);
        t = Integer.parseInt(line[1]);
        String s1 = input.readLine();
        String s2 = input.readLine();
        char[] s3 = new char[n];
        int a, b = 0;
        Arrays.fill(s3, 'A');
        ArrayList<Integer> com = new ArrayList<Integer>();
        HashSet<Integer> set = new HashSet<Integer>();
        for(int i = 0; i < n; i++){
            if(s1.charAt(i) == s2.charAt(i)){
                b++;
                set.add(i);
                com.add(i);
            }
        }
        a = n - b;
        if(n - t  > b + a/2){
            pw.println(-1);
        }else{
            if(n - t <= b){
                for(int i = 0; i < n - t; i++){
                    s3[com.get(i)] = s1.charAt(com.get(i));
                }
            }else{
                for(int i = 0; i < b; i++){
                    s3[com.get(i)] = s1.charAt(com.get(i));
                }
                //n-t-b
                boolean one = true;
                
                for(int i = 0,j = 0; i < n && j < 2*(n-t-b); i++){
                    if(!set.contains(i)){
                        if(one){
                            s3[i] = s1.charAt(i);
                            j++;
                        }else{
                            s3[i] = s2.charAt(i);
                            j++;
                        }
                    }
                    if(j == n - t- b){
                        one = false;
                    }
                }
                
            }
            for(int i = 0; i < n; i++){
                if(s3[i] == 'A'){
                    s3[i] = getR2(s1.charAt(i), s2.charAt(i));
                }
            }
            StringBuilder ans = new StringBuilder("");
            for(int i = 0; i < n; i++){
                
                
                ans.append(s3[i]);
            }
            pw.println(ans);
        }
        pw.close();
        input.close();
    }
}