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 CPP):

Solution:
#include <bits/stdc++.h>
int n, t;
char r[100013], s[100013];
char ans[1000013];
int main() {
  scanf("%d%d", &n, &t);
  t = n - t;
  scanf("%s%s", r, s);
  int slen = strlen(s);
  int check = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == r[i]) {
      check++;
      ans[i] = s[i];
    } else
      ans[i] = 0;
  }
  ans[n] = 0;
  if (check > t) {
    for (int i = 0; i < n; i++) {
      if (s[i] == r[i] && t > 0) {
        printf("%c", s[i]);
        t--;
      } else {
        char out = 'a';
        while (out == s[i] || out == r[i]) out++;
        printf("%c", out);
      }
    }
    printf("\n");
  } else {
    int t1, t2;
    t1 = t2 = t - check;
    for (int i = 0; i < n; i++) {
      if (ans[i] == 0) {
        if (t1 >= t2 && t1 > 0) {
          ans[i] = s[i];
          t1--;
        } else if (t1 < t2 && t2 > 0) {
          ans[i] = r[i];
          t2--;
        } else {
          char out = 'a';
          while (out == s[i] || out == r[i]) out++;
          ans[i] = out;
        }
      }
    }
    if (t1 != 0 || t2 != 0)
      printf("-1");
    else
      printf("%s\n", ans);
  }
  scanf("\n");
  return 0;
}