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.StringTokenizer;

/**
 * Created by peacefrog on 12/6/15.
 * Time : 5:31 PM
 */

public class CF584C {
	final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
	PrintWriter out;
	long timeBegin, timeEnd;

	public void runIO() throws IOException {
		timeBegin = System.currentTimeMillis();

		InputStream inputStream;
		OutputStream outputStream;


		if (ONLINE_JUDGE) {
			inputStream = System.in;
			Reader.init(inputStream);
			outputStream = System.out;

			out = new PrintWriter(outputStream);

		} else {
			inputStream = new FileInputStream("/home/peacefrog/Dropbox/IdeaProjects/Problem Solving/input");
			Reader.init(inputStream);
			out = new PrintWriter(System.out);
		}

		solve();

		out.flush();
		out.close();
		timeEnd = System.currentTimeMillis();

		System.err.println("Time = " + (timeEnd - timeBegin));

	}

	/*
	 * Start Solution Here
	 */

	private void solve() throws IOException {
		int n = Reader.nextInt(); //This Variable default in Code Template
		int t = Reader.nextInt();

		char [] s1 = Reader.next().toCharArray();
		char [] s2 = Reader.next().toCharArray();

		char [] s = new char[n];

		int same = 0 ;
		for (int i = 0; i < n; i++) {
			if(t== 0 && s1[i] != s2[i])
			{
				out.println(-1);
				return;
			}
		}
		for (int i = 0; i < n; i++) {
			s[i] = s1[i];
			if(s1[i] == s2[i] )
			{
				same++;
			}
		}
		if(2*t < (n-same))
		{
			out.println(-1);
			return;
		}
		if(n - t <= same) {
			int changeSame = t- (n -same);
			for (int i = 0; i < n; i++) {
				if (s1[i] != s2[i] &&  t>0 ) {
					for (char c= 'a' ; c<= 'z' ;c++ )
					{
						if (c != s1[i] && c!= s2[i])
						{
							s[i] = c;
							break;
						}
					}
					t--;
				}
				else if(s1[i]==s2[i] && changeSame>0)
				{
					for (char c= 'a' ; c<= 'z' ;c++ )
					{
						if (c != s1[i] )
						{
							s[i] = c;
							break;
						}
					}
					changeSame--;
					t--;
				}
			}
			out.println(s);
		}
		/*else if(same >=t)
		{
			for (int i = 0; i < n; i++) {
				if(s1[i] == s2[i] && t>0)
				{
					for (char c= 'a' ; c<= 'z' ;c++ )
					{
						if (c != s1[i])
						{
							s[i] = c;
							break;
						}
					}
					t--;
				}
			}
			out.println(s);
		}*/
		else {

			int difference = n - same ;
			int again = difference- t;
//			System.out.println(extra);
			int first = 0, second = difference;
			for (int i = 0; i < n; i++) {
				if (s1[i] != s2[i]) {
					if (first < again) {
						s[i] = s1[i];
					} else if (second <= again) {
						s[i] = s2[i];
					}
					else
					{
						for (char c= 'a' ; c<= 'z' ;c++ )
						{
							if (c != s1[i] && c!= s2[i])
							{
								s[i] = c;
								break;
							}
						}
					}
					first++;
					second--;
				}
				/*else if (s1[i] == s2[i] && again==0) {
					for (char c= 'a' ; c<= 'z' ;c++ )
					{
						if (c != s1[i] && c!= s2[i])
						{
							s[i] = c;
							break;
						}
					}
				}*/
				/*else if(same>0 )
				{
					for (char c= 'a' ; c<= 'z' ;c++ )
					{
						if (c != s1[i])
						{
							s[i] = c;
							break;
						}
					}
					same--;
				}*/
			}
			out.println(s);
		}
	}

	public static void main(String[] args) throws IOException {

		new CF584C().runIO();
	}

	static class Reader {
		static BufferedReader reader;
		static StringTokenizer tokenizer;

		/**
		 * call this method to initialize reader for InputStream
		 */
		static void init(InputStream input) {
			reader = new BufferedReader(new InputStreamReader(input));
			tokenizer = new StringTokenizer("");
		}

		/**
		 * get next word
		 */
		static String next() {
			while (tokenizer == null || !tokenizer.hasMoreTokens()) {
				try {
					tokenizer = new StringTokenizer(reader.readLine());
				} catch (IOException e) {
					throw new RuntimeException(e);
				}
			}
			return tokenizer.nextToken();
		}

		static String nextLine() {
			try {
				return reader.readLine();
			} catch (IOException e) {
				e.printStackTrace();
			}
			return "";
		}

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

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

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

		static long[] nextLongArray(int n) throws IOException {
			long[] arr = new long[n];
			for (int i = 0; i < n; i++) {
				arr[i] = nextLong();
			}

			return arr;
		}

		static int[] nextIntArray(int n) throws IOException {
			int[] arr = new int[n];
			for (int i = 0; i < n; i++) {
				arr[i] = nextInt();
			}

			return arr;
		}
	}

}