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.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.*;

public class Main {

    public static void solve() {
        int n = IO.readInt();
        int t = IO.readInt();

        String s1 = IO.readString();
        String s2 = IO.readString();

        int d = 0;
        for (int index = 0; index < n; ++index) {
            if (s1.charAt(index) != s2.charAt(index)) {
                d++;
            }
        }

        int ca = 0;
        int cd = 0;
        int t1 = 0;
        int t2 = 0;

        if (d > 2 * t) {
            IO.println(-1);
            return;
        }
        else if (d < t) {
            ca = t - d;
            cd = d;
        }
        else {
            cd = 2 * t - d;
            t1 = t - cd;
            t2 = t1;
        }

        StringBuilder sb = new StringBuilder();
        for (int index = 0; index < n; ++index) {
            char c1 = s1.charAt(index);
            char c2 = s2.charAt(index);
            char toAppend;
            if (c1 == c2) {
                if (ca-- > 0) {
                    toAppend = getAnother(c1, c2);
                }
                else {
                    toAppend = c1;
                }
            }
            else {
                if (t1-- > 0) {
                    toAppend = c1;
                }
                else if (t2-- > 0) {
                    toAppend = c2;
                }
                else {
                    toAppend = getAnother(c1, c2);
                }
            }
            sb.append(toAppend);
        }

        IO.println(sb);
    }

    static char getAnother(char... s) {
        loop : for (char c = 'a'; c < 'd'; ++c) {
            for (char ss : s) {
                if (ss == c) {
                    continue loop;
                }
            }
            return c;
        }
        return 'd';
    }


    // Input/Output ================================
    public static class IO {

        private static Scanner scanner;
        private static PrintWriter writer;

        public static void setUp(InputStream in, OutputStream out) {
            scanner = new Scanner(new java.io.BufferedInputStream(in));
            writer = new PrintWriter(out);
        }

        public static boolean isEmpty() {
            return !scanner.hasNext();
        }

        public static boolean hasNextLine() {
            return scanner.hasNextLine();
        }

        public static String readLine() {
            String line;
            try {
                line = scanner.nextLine();
            } catch (Exception e) {
                line = null;
            }
            return line;
        }

        public static String readString() {
            return scanner.next();
        }

        public static int readInt() {
            return scanner.nextInt();
        }

        public static int[] readIntArray(int n) {
            int[] a = new int[n];
            for (int index = 0; index < n; ++index) {
                a[index] = readInt();
            }
            return a;
        }

        public static double readDouble() {
            return scanner.nextDouble();
        }

        public static float readFloat() {
            return scanner.nextFloat();
        }

        public static long readLong() {
            return scanner.nextLong();
        }

        public static char readChar() {
            return scanner.findInLine("[^\\s]").charAt(0);
        }

        public static void print(Object o) {
            writer.print(o);
            writer.flush();
        }

        public static void println(Object o) {
            writer.println(o);
            writer.flush();
        }
    }

    // Algorithms ================================

    private static class Algo {

        private static int binSearch(int[] array, int n, int lo, int hi) {
            if (lo > hi) {
                return lo;
            }

            int mid = lo + (hi - lo) / 2;
            int cmp = (int) java.lang.Math.signum(n - array[mid]);
            if (cmp < 0) {
                return binSearch(array, n, lo, mid - 1);
            } else if (cmp == 0) {
                return mid;
            } else {
                return binSearch(array, n, mid + 1, hi);
            }
        }

        private static void permutations(int n) {
            int[] p = new int[n];
            for (int index = 0; index < n; ++index) {
                p[index] = index + 1;
            }

            while (true) {
                IO.println(Arrays.toString(p));

                int k = n - 1;
                while (k > 0) {
                    if (p[k] < p[k - 1]) {
                        k--;
                    }
                    else {
                        break;
                    }
                }
                if (k == 0) {
                    break;
                }
                int j = n - 1;
                while (p[j] < p[k - 1]) {
                    j--;
                }

                int t = p[j];
                p[j] = p[k - 1];
                p[k - 1] = t;

                int i = 0;
                while (k + i < n - 1 - i) {
                    t = p[k + i];
                    p[k + i] = p[n - i - 1];
                    p[n - i - 1] = t;
                    i++;
                }
            }
        }

        private static boolean isPrime(int number) {
            if (number <= 1) {
                return false;
            }
            if (number <= 3) {
                return true;
            }
            if (number % 2 == 0 || number % 3 == 0) {
                return false;
            }
            int i = 5;
            while (i * i <= number) {
                if (number % i == 0 || number % (i + 2) == 0) {
                    return false;
                }
                i += 6;
            }
            return true;
        }

        private static boolean isPalindrome(int number) {
            int n = number;
            int reverse = 0;
            while (n > 0) {
                reverse *= 10;
                reverse += (n % 10);
                n /= 10;
            }
            return reverse == number;
        }

        public static int gcd(int a,int b) {
            while (b != 0) {
                int tmp = a % b;
                a = b;
                b = tmp;
            }
            return a;
        }
    }

    private static class Math2 {

        static long c(int n, int k) {
            if (n == 0 && k == 0) {
                return 1;
            }
            int d = 1000;
            int l = 1000000007;
            long[][] c = new long[d + 1][d + 1];
            for (int i = 1; i <= d; ++i) {
                for (int j = 0; j <= i; ++j) {
                    if (i == j || j == 0) {
                        c[i][j] = 1;
                    }
                    else {
                        c[i][j] = (c[i - 1][j - 1] + c[i - 1][j]) % l;
                    }
                }
            }
            return c[n][k];
        }
    }

    public static void main(String[] args) {
        IO.setUp(System.in, System.out);
        /*
        try {
            IO.setUp(new FileInputStream("encode.in"), new FileOutputStream("encode.out"));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
            return;
        }
        */
        solve();
    }
}