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.util.*;
import java.io.*;
import java.math.*;
public class Main1
{
    static class Reader 
    { 
        private InputStream mIs;private byte[] buf = new byte[1024];private int curChar,numChars;public Reader() { this(System.in); }public Reader(InputStream is) { mIs = is;} 
        public int read() {if (numChars == -1) throw new InputMismatchException();if (curChar >= numChars) {curChar = 0;try { numChars = mIs.read(buf);} catch (IOException e) { throw new InputMismatchException();}if (numChars <= 0) return -1; }return buf[curChar++];} 
        public String nextLine(){int c = read();while (isSpaceChar(c)) c = read();StringBuilder res = new StringBuilder();do {res.appendCodePoint(c);c = read();}while (!isEndOfLine(c));return res.toString() ;} 
        public String s(){int c = read();while (isSpaceChar(c)) c = read();StringBuilder res = new StringBuilder();do {res.appendCodePoint(c);c = read();}while (!isSpaceChar(c));return res.toString();} 
        public long l(){int c = read();while (isSpaceChar(c)) c = read();int sgn = 1;if (c == '-') { sgn = -1 ; c = read() ; }long res = 0; do{ if (c < '0' || c > '9') throw new InputMismatchException();res *= 10 ; res += c - '0' ; c = read();}while(!isSpaceChar(c));return res * sgn;} 
        public int i(){int c = read() ;while (isSpaceChar(c)) c = read();int sgn = 1;if (c == '-') { sgn = -1 ; c = read() ; }int res = 0;do{if (c < '0' || c > '9') throw new InputMismatchException();res *= 10 ; res += c - '0' ; c = read() ;}while(!isSpaceChar(c));return res * sgn;} 
        public double d() throws IOException {return Double.parseDouble(s()) ;}
        public boolean isSpaceChar(int c) { return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1; } 
        public boolean isEndOfLine(int c) { return c == '\n' || c == '\r' || c == -1; } 
    }
    
    
    ///////////////////////////////////////////////////////////////////////////////////////////
    //    RRRRRRRRR            AAA             HHH     HHH     IIIIIIIIIIIII   LLL           //
    //    RR    RRR           AAAAA            HHH     HHH      IIIIIIIIIII    LLL           //
    //    RR   RRR           AAAAAAA           HHH     HHH          III        LLL           //
    //    RR  RRR           AAA   AAA          HHHHHHHHHHH          III        LLL           //
    //    RRRRRR           AAA     AAA         HHHHHHHHHHH          III        LLL           //
    //    RR  RRR         AAAAAAAAAAAAA        HHH     HHH          III        LLL           //
    //    RR    RRR      AAA         AAA       HHH     HHH      IIIIIIIIIII    LLLLLLLLLLLL  //
    //    RR     RRR    AAA           AAA      HHH     HHH     IIIIIIIIIIIII   LLLLLLLLLLLL  //
    ///////////////////////////////////////////////////////////////////////////////////////////
    
    public static void main(String[] args)throws IOException
    {
        PrintWriter out= new PrintWriter(System.out);
        Reader sc=new Reader();
        int n=sc.i();
        int t=sc.i();
        String s1=sc.s();
        String s2=sc.s();
        int q=0;
        for(int i=0;i<n;i++)if(s1.charAt(i)==s2.charAt(i))q++;
        int k=n-t;
        if(n-q<2*(k-q))
        {
            System.out.println(-1);
            System.exit(0);
        }
        if(k<=q)
        {
            int count=0;
            for(int i=0;i<n;i++)
                if(s1.charAt(i)==s2.charAt(i)&&count!=k)
                {
                    count++;
                    out.print(s1.charAt(i));
                }
                else
                {
                    int mark[]=new int[26];
                    mark[s1.charAt(i)-97]++;mark[s2.charAt(i)-97]++;
                    for(int j=0;j<26;j++)
                    {
                        if(mark[j]==0)
                        {
                            out.print((char)(j+97));
                            break;
                        }
                    }
                }
            out.flush();
            System.exit(0);
        }
        k-=q;
        int count1=0;
        int count2=0;
        for(int i=0;i<n;i++)
        {
            if(s1.charAt(i)==s2.charAt(i))
            out.print(s1.charAt(i));
            else
            {
                if(count1!=k)
                {
                    out.print(s1.charAt(i));
                    count1++;
                }
                else if(count2!=k)
                {
                    out.print(s2.charAt(i));
                    count2++;
                }
                else
                {
                    int mark[]=new int[26];
                    mark[s1.charAt(i)-97]++;mark[s2.charAt(i)-97]++;
                    for(int j=0;j<26;j++)
                    {
                        if(mark[j]==0)
                        {
                            out.print((char)(j+97));
                            break;
                        }
                    }
                }
            }
        }
        out.flush();
    }
}