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.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
import java.lang.*;

public class Third {
    //static long m=1000000007;
    static BigInteger ways(int N,  int K) {
        BigInteger ret = BigInteger.ONE;
        for(int i=N;i>=N-K+1;i--)
        {
             ret = ret.multiply(BigInteger.valueOf(i));
            
        }
        for (int j = 1; j<=K; j++) {
            ret = ret.divide(BigInteger.valueOf(j));
        }
       
            ret=ret.mod(BigInteger.valueOf(1000000007));
        
        return ret;
    }
    public static int prime(int n)
    {
        int f=1;
        if(n==1)
            return 0;
        for(int i=2;i<=(Math.sqrt(n));)
        {
            if(n%i==0)
            {
                f=0;
                break;
            }
            if(i==2)
                i++;
            else
                i+=2;
        }
        if(f==1)
            return 1;
        else 
            return 0;
    }
/*public static long gcd(long x,long y)
{
    if(x%y==0)
        return y;
    else return gcd(y,x%y);
}*/
    public static BigInteger fact(int n)
    {
        BigInteger f=BigInteger.ONE;
        for(int i=1;i<=n;i++)
        {
            f=f.multiply(BigInteger.valueOf(i));
        }
        //f=f.mod(BigInteger.valueOf(m));
        return f;
    }
public static int gcd(int x,int y)
{
    if(x%y==0)
        return y;
    else return gcd(y,x%y);
}
public static int max(int a,int b)
{
    if(a>b)
        return a;
    else
        return b;
}
public static int min(int a,int b)
{
    if(a>b)
        return b;
    else 
        return a;
}
static long ncr(long n,long k)
{
    long w=1;
    for(long i=n,j=1;i>=n-k+1;i--,j++)
    {
        w=w*i;
        w=w/j;
        w%=1000000007;
        
    }
    return w;
}
static class Pair implements Comparable<Pair>
{
    int a,b;
    Pair (int a,int b)
    {
        this.a=a;
        this.b=b;
    }

    public int compareTo(Pair o) {
        // TODO Auto-generated method stub
        if(this.a!=o.a)
        return Integer.compare(this.a,o.a);
        else
            return Integer.compare(this.b, o.b);
        //return 0;
    }
}

    public static void main(String[] args) throws Exception{
        
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        StringBuilder sb=new StringBuilder("");
         //InputReader in = new InputReader(System.in);
        // OutputWriter out = new OutputWriter(System.out);
        //PrintWriter pw=new PrintWriter(System.out);
        //String line=br.readLine().trim();
        //int t=Integer.parseInt(br.readLine());
    //  while(t-->0)
        //{
        //int n=Integer.parseInt(br.readLine());
        //long n=Long.parseLong(br.readLine());
        //line=br.readLine().trim();
        //String l[]=br.readLine().split(" ");
        //int m=Integer.parseInt(l[0]);
        //int k=Integer.parseInt(l[1]);
        //l=br.readLine().split(" ");
        //long m=Long.parseLong(l[0]);
        //long n=Long.parseLong(l[1]);
        //char ch=a.charAt();
    //  char c[]=new char[n];
        //String l[]=br.readLine().split(" ");
        //l=br.readLine().split(" ");
        /*int a[]=new int[n];
        for(int i=0;i<n;i++)
        {
            a[i]=Integer.parseInt(l[i]);
        }*/
        /*long a[]=new long[n];
        for(int i=0;i<n;i++)
        {
            a[i]=Long.parseLong(l[i]);
        }*/
        /*int a[][]=new int[n][n];
        for(int i=0;i<n;i++)
        {
        l=br.readLine().split(" ");
            for(int j=0;j<n;j++)
            {
                a[i][j]=Long.parseLong(l[j]);
            }
        }*/
        
        //  String a=br.readLine();
        
        // char a[]=c.toCharArray();
        //char ch=l[0].charAt(0);
        
        //HashMap<Integer,Integer>hm=new HashMap<Integer,Integer>();
        //HashMap<Integer,String>hm=new HashMap<Integer,String>();
        //HashMap<Integer,Long>hm=new HashMap<Integer,Long>();
        //hm.put(1,1);
        //HashSet<Integer>hs=new HashSet<Integer>();
        //HashSet<Long>hs=new HashSet<Long>();
        //HashSet<String>hs=new HashSet<String>();
        //hs.add(x);
        //Stack<Integer>s=new Stack<Integer>();
        //s.push(x);
        //s.pop(x);
        //Queue<Integer>q=new LinkedList<Integer>();
        //q.add(x);
        //q.remove(x);
        //ArrayList<Integer>ar=new ArrayList<Integer>();
        //long x=Long.parseLong(l[0]);
        
        //int min=100000000;
    
        //long c1=(long)Math.ceil(n/(double)a);
        //Arrays.sort(a);
          // int f1[]=new int[26];
           //int f2[]=new int[26];
        /*for(int i=0;i<n;i++)
        {
        int x=a.charAt(0);
            f1[x-97]++;
        }*/
        /*for(int i=0;i<n;i++)
        {
            
        }*/
        /*for(int i=0;i<n;i++)
        {
            for(int j=0;j<n;j++)
            {
                
            }
        }*/
        /*while(q--)
          {
                l=br.readLine().split(" ");
                int n=Integer.parseInt(l[0]);
                int k=Integer.parseInt(l[1]);
          }
         */
        /*if(f==1)
            System.out.println("Yes");
        else
            System.out.println("No");*/
        //System.out.print("");
        //sb.append("");
        //sb.append("").append("\n");
        
        String l[]=br.readLine().split(" ");
        int n=Integer.parseInt(l[0]);
        int t=Integer.parseInt(l[1]);
        String a=br.readLine();
        String b=br.readLine();
        char c[]=a.toCharArray();
        char d[]=b.toCharArray();
        int g[]=new int[26];
        int h[]=new int[26];
      //    Arrays.sort(c);
     // Arrays.sort(d);
        char e[]=new char[c.length];
        for(int i=0;i<c.length;i++)
        {
            g[c[i]-97]++;
        }
        for(int i=0;i<c.length;i++)
        {
            h[d[i]-97]++;
        }
        int f=1;
        if(c.length!=d.length)
            f=0;
        else
        {
        /*for(int i=0;i<26;i++)
        {
            
        }*/
            int c1=0;
        for(int i=0;i<c.length;i++)
        {
        if(c[i]!=d[i])
            c1++;
        }
        int ds=(int)Math.ceil(c1/2.0);
        if(t>=ds)
        {
            int j=1;
            /*for(int i=0;i<c.length;i++)
            {
                if(c[i]==d[i])
                {
                    e[i]=c[i];
                }
                else
                {
                    if(c[i]=='a')
                    {
                        if(d[i]=='b')
                            e[i]='c';
                        else
                            e[i]='b';
                    }
                    else if(d[i]=='a')
                    {
                        if(c[i]=='b')
                            e[i]='c';
                        else
                            e[i]='b';
                    }
                    else
                        e[i]='a';
                }
            }*/
            int s=n-t;
            for(int i=0;i<c.length;i++)
            {
                if(c[i]==d[i]&&s>0)
                {
                    e[i]=c[i];
                    //System.out.println(e[i]);
                    s--;
                }
            }
            int sf=s;
            //int df=(int)Math.ceil(s/2.0);
            int df=s;
            for(int i=0;i<n;i++)
            {
                if(e[i]=='\0'&&sf>0)
                {
                    e[i]=c[i];
                    sf--;
                }
            }
            for(int i=0;i<n;i++)
            {
                if(e[i]=='\0'&&df>0)
                {
                    e[i]=d[i];
                    df--;
                }
            }
            for(int i=0;i<n;i++)
            {
                if(e[i]=='\0')
                {
                if(c[i]=='a')
                {
                    if(d[i]=='b')
                        e[i]='c';
                    else
                        e[i]='b';
                }
                else if(d[i]=='a')
                {
                    if(c[i]=='b')
                        e[i]='c';
                    else
                        e[i]='b';
                }
                else
                    e[i]='a';
            }
            }
        }
        else
            f=0;
        }
        if(f==0)
         System.out.println("-1");   
        else
        {
            for(int i=0;i<c.length;i++)
           System.out.print(e[i]);          
        
        }
    }
   static  class InputReader
    {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream)
        {
            this.stream = stream;
        }

        public int read()
        {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars)
            {
                curChar = 0;
                try
                {
                    numChars = stream.read(buf);
                } catch (IOException e)
                {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int readInt()
        {
            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 String readString()
        {
            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 double readDouble() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, readInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, readInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }
        public long readLong() {
            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 boolean isSpaceChar(int c)
        {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next()
        {
            return readString();
        }

        public interface SpaceCharFilter
        {
            public boolean isSpaceChar(int ch);
        }
    }

     static class OutputWriter
    {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream)
        {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer)
        {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects)
        {
            for (int i = 0; i < objects.length; i++)
            {
                if (i != 0)
                    writer.print(' ');
                writer.print(objects[i]);
            }
        }

        public void printLine(Object... objects)
        {
            print(objects);
            writer.println();
        }

        public void close()
        {
            writer.close();
        }

        public void flush()
        {
            writer.flush();
        }

    }
    /* USAGE

    //initialize
        InputReader in      = new InputReader(System.in);
        OutputWriter out    =   new OutputWriter(System.out);

    //read int
    int i = in.readInt();
    //read string
    String s = in.readString();
    //read int array of size N
    int[] x = IOUtils.readIntArray(in,N);
    //printline
    out.printLine("X");


    //flush output
    out.flush();

    //remember to close the
    //outputstream, at the end
    out.close();

    */
  static   class IOUtils {

        public static int[] readIntArray(InputReader in, int size) {
            int[] array = new int[size];
            for (int i = 0; i < size; i++)
                array[i] = in.readInt();
            return array;
    }
    }
}