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.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;

public class C {

    static StringTokenizer st;
    static BufferedReader in;
    static PrintWriter pw;
    public static void main(String[] args) throws IOException{
        in = new BufferedReader(new InputStreamReader(System.in));
        pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        int n = nextInt();
        int t = nextInt();
        t = n - t;
        char[]s1 = next().toCharArray();
        char[]s2 = next().toCharArray();
        char[]s = new char[n];
        Arrays.fill(s, ' ');
        int t1 = t, t2 = t;
        for (int i = 0; i < n; i++) {
            if (s1[i]==s2[i]) {
                if (t1 > 0 && t2 > 0) {
                    t1--;
                    t2--;
                    s[i] = s1[i];
                }
                else {
                    for (char j = 'a'; j <= 'z'; j++) {
                        if (j != s1[i]) {
                            s[i] = j;
                            break;
                        }
                    }
                }
            }
        }
        for (int i = 0; i < n; i++) {
            if (s1[i] != s2[i]) {
                if (t1 > 0) {
                    t1--;
                    s[i] = s1[i];
                }
                else if (t2 > 0) {
                    s[i] = s2[i];
                    t2--;
                }
                else {
                    for (char j = 'a'; j <= 'z'; j++) {
                        if (j != s1[i] && j != s2[i]) {
                            s[i] = j;
                            break;
                        }
                    }
                }
            }
        }
        if (t1 != 0 || t2 != 0) {
            System.out.println(-1);
            return;
        }
        for (int i = 0; i < n; i++) {
            pw.print(s[i]);
        }
        pw.close();
    }
    private static int nextInt() throws IOException{
        return Integer.parseInt(next());
    }
    private static double nextDouble() throws IOException{
        return Double.parseDouble(next());
    }
    private static long nextLong() throws IOException{
        return Long.parseLong(next());
    }
    private static String next() throws IOException {
        while (st == null || !st.hasMoreTokens()) {
            st = new StringTokenizer(in.readLine());
        }
        return st.nextToken();
    }
}