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 Main {
	
	static InputReader in = new InputReader(System.in);
	static PrintWriter out = new PrintWriter(System.out);

	static int MOD = 1000000007;
	static int M = 505;
	static int oo = (int)1e9;
	
	
	public static void main(String[] args) throws IOException {

//		while(true) {
			int n = in.nextInt();
//			int n = 3;
			int t = in.nextInt();
//			int t = 2;
			char[] a = in.readString().toCharArray();
			char[] b = in.readString().toCharArray();
//			char[] a = new char[n];
//			char[] b = new char[n];
//			for(int i=0; i < n; ++i) {
//				Random r = new Random();
//				a[i] = (char)('a' + r.nextInt(26));
//				b[i] = (char)('a' + r.nextInt(26));
//			}
			
			ArrayList<Integer> diff = new ArrayList<>();
			ArrayList<Integer> same = new ArrayList<>();
			for(int i=0; i<n; ++i) {
				if(a[i] == b[i])
					same.add(i);
				else
					diff.add(i);
			}
			
			char[] c = new char[n];
			int da = 0, db = 0, cnt = 0;
			for(int k=0; k < diff.size(); ++k) {
				int i = diff.get(k);
				if(da > db) {
					c[i] = a[i];
					db++;
				}
				else if(da == t) {
					System.out.println(-1);
					return;
				}
				else if(k == diff.size() - 1 || t - da == n - cnt) {
					for(char ch = 'a'; ch <= 'z'; ++ch) {
						if(ch != a[i] && ch != b[i]) {
							c[i] = ch;
							break;
						}
					}
					da++;
					db++;
				}
				else {
					c[i] = b[i];
					da++;
				}
				cnt++;
			}
			
			for(int k=0; k < same.size(); ++k) {
				int i = same.get(k);
				if(da == t) {
					c[i] = a[i];
				}
				else {
					c[i] = (char)(a[i] + 1);
					if(c[i] > 'z')
						c[i] = 'a';
					da++;
					db++;
				}
			}
			
//			System.out.println(String.valueOf(a));
//			System.out.println(String.valueOf(b));
			System.out.println(String.valueOf(c));
			
//			if(f(a, c) != f(b, c)) {
//				System.out.println(String.valueOf(a));
//				System.out.println(String.valueOf(b));
//				System.out.println(String.valueOf(c));
//				break;
//			}
//		}

		
		out.close();
	}
	
	static int f(char[] a, char[] b) {
		int ret = 0;
		for(int i=0; i < a.length; ++i) {
			if(a[i] != b[i])
				ret++;
		}
		return ret;
	}
	
	static void shuffle(int[] a) {
		Random r = new Random();
		for(int i = a.length - 1; i > 0; --i) {
			int si = r.nextInt(i);
			int t = a[si];
			a[si] = a[i];
			a[i] = t;
		}
	}
	
	static void shuffle(long[] a) {
		Random r = new Random();
		for(int i = a.length - 1; i > 0; --i) {
			int si = r.nextInt(i);
			long t = a[si];
			a[si] = a[i];
			a[i] = t;
		}
	}
	
	static int lower_bound(int[] a, int n, int k) {
		int s = 0;
		int e = n;
		int m;
		while (e - s > 0) {
			m = (s + e) / 2;
			if (a[m] < k)
				s = m + 1;
			else
				e = m;
		}
		return e;
	}
	static int lower_bound(long[] a, int n, long k) {
		int s = 0;
		int e = n;
		int m;
		while (e - s > 0) {
			m = (s + e) / 2;
			if (a[m] < k)
				s = m + 1;
			else
				e = m;
		}
		return e;
	}
	
	static int gcd(int a, int b) {
		return b == 0 ? a : gcd(b, a % b);
	}
	static long gcd(long a, long b) {
		return b == 0 ? a : gcd(b, a % b);
	}
	
	static class Pair implements Comparable<Pair> {
		int first, second;

		public Pair(int first, int second) {
			super();
			this.first = first;
			this.second = second;
		}

		@Override
		public int compareTo(Pair o) {
			return this.first != o.first ? this.first - o.first : this.second - o.second;
		}

		@Override
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + first;
			result = prime * result + second;
			return result;
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			Pair other = (Pair) obj;
			if (first != other.first)
				return false;
			if (second != other.second)
				return false;
			return true;
		}
	}
	
	
}



class InputReader {

	private final InputStream stream;
	private final byte[] buf = new byte[8192];
	private int curChar, snumChars;

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

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

	public int nextInt() {
		int c = read();
		while (isSpaceChar(c)) {
			c = read();
		}
		int sgn = 1;
		if (c == '-') {
			sgn = -1;
			c = read();
		}
		int res = 0;
		do {
			res *= 10;
			res += c - '0';
			c = read();
		} while (!isSpaceChar(c));
		return res * sgn;
	}

	public long nextLong() {
		int c = read();
		while (isSpaceChar(c)) {
			c = read();
		}
		int sgn = 1;
		if (c == '-') {
			sgn = -1;
			c = read();
		}
		long res = 0;
		do {
			res *= 10;
			res += c - '0';
			c = read();
		} while (!isSpaceChar(c));
		return res * sgn;
	}

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

	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 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 boolean isSpaceChar(int c) {
		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
	}

	private boolean isEndOfLine(int c) {
		return c == '\n' || c == '\r' || c == -1;
	}

}