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>
using namespace std;
int main() {
  int n, t;
  scanf("%d%d", &n, &t);
  char s1[n + 1];
  char s2[n + 1];
  scanf(" %s", s1);
  scanf(" %s", s2);
  char ans[n + 1];
  for (int i = 0; i < n; i++) {
    ans[i] = '.';
  }
  int temp = n - t;
  for (int i = 0; i < n; i++) {
    if (temp == 0) break;
    if (s1[i] == s2[i]) {
      ans[i] = s1[i];
      temp--;
    }
  }
  int f = 0;
  for (int i = 0; i < n; i++) {
    if (ans[i] == '.') {
      if (temp != 0 && f == 0) {
        ans[i] = s1[i];
        f = 1;
      } else if (temp != 0 && f == 1) {
        ans[i] = s2[i];
        f = 0;
        temp--;
      } else {
        int cc = (int)s1[i] + 1;
        int dd = (int)s2[i] + 1;
        if (cc > 122) cc = 97;
        if (dd > 122) dd = 97;
        char c = (char)cc;
        char d = (char)dd;
        if (c != s2[i]) {
          ans[i] = c;
        } else if (d != s1[i]) {
          ans[i] = d;
        }
      }
    }
  }
  for (int i = 0; i < n; i++) {
    int temp = (int)ans[i];
    if (temp < 97 || temp > 122) {
      printf("%d %c", i, temp);
    }
  }
  ans[n] = '\0';
  if (temp != 0)
    printf("-1\n");
  else
    printf("%s", ans);
  return 0;
}