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.*;
import java.util.*;

public class tank {

    static final FastScanner fs = new FastScanner();

    //static PrintWriter out = new PrintWriter(System.out);

    public static void main(String[] args) {
        int t = 1;
        while(t-->0) {
            run_case();
        }

        //out.close();
    }

    static void run_case() {
        int n = fs.nextInt(), t = fs.nextInt(), same = n - t;
        char[] a = fs.next().toCharArray();
        char[] b = fs.next().toCharArray();
        char[] ans = new char[n];

        Arrays.fill(ans, 'A');

        for (int i = 0; i < n; i++) {
            if(a[i] == b[i]) {
                if(same > 0) {
                    same--;
                    ans[i] = a[i];
                }else break;
            }
        }

        int ta = t, tb = t;

        if(same > 0) {
            int sa = same, sb = sa;
            for (int i = 0; i < n; i++) {
                if(a[i] != b[i]) {
                    if(sa > 0) {
                        sa--;
                        ans[i] = a[i];
                        tb--;
                    }else if(sb > 0) {
                        sb--;
                        ans[i] = b[i];
                        ta--;
                    }else break;
                }

                if(ta < 0 || tb < 0) {
                    System.out.println(-1);
                    return;
                }
            }

            if(sb > 0) {
                System.out.println(-1);
                return;
            }
        }

        for (int i = 0; i < n; i++) {
            if(ans[i] == 'A') {
                ta--;
                if(a[i] == b[i]) {
                    if(a[i] == 'a') ans[i] = 'b';
                    else ans[i] = 'a';
                }else{
                    if(a[i] != 'z' && b[i] != 'z') ans[i] = 'z';
                    else if(a[i] != 'y' && b[i] != 'y') ans[i] = 'y';
                    else if(a[i] != 'x' && b[i] != 'x') ans[i] = 'x';
                    else if(a[i] != 'u' && b[i] != 'u') ans[i] = 'u';
                    else ans[i] = 'v';
                }
            }
        }

        if(ta != 0) {
            System.out.println(-1);
        }else{
            System.out.println(new String(ans));
        }
    }

    static class FastScanner {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st=new StringTokenizer("");
        String next() {
            while (!st.hasMoreTokens())
                try {
                    st=new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            return st.nextToken();
        }

        String nextLine(){
            try {
                return br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }

            return "";
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        int[] readArray(int n) {
            int[] a=new int[n];
            for (int i=0; i<n; i++) a[i]=nextInt();
            return a;
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }
    }
}