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;
char ans[100005];
int main() {
  int n, k;
  cin >> n >> k;
  string s1, s2;
  cin >> s1 >> s2;
  k = n - k;
  for (int i = 0; i <= 100000; i++) {
    ans[i] = '!';
  }
  int sum1 = 0;
  int sum2 = 0;
  for (int i = 0; i < n; i++) {
    if (sum1 == k) {
      break;
    }
    if (s1[i] == s2[i]) {
      ans[i] = s1[i];
      sum1++;
      sum2++;
    }
  }
  int last = 2;
  for (int i = 0; i < n; i++) {
    if (sum1 == k && sum2 == k) break;
    if (ans[i] != '!') continue;
    if (last == 2 && sum1 < k) {
      ans[i] = s1[i];
      sum1++;
      if (sum2 < k) last = 1;
    } else if (last == 1 && sum2 < k) {
      ans[i] = s2[i];
      sum2++;
      if (sum1 < k) last = 2;
    }
  }
  if (sum1 < k || sum2 < k) {
    cout << -1;
    return 0;
  }
  for (int i = 0; i < n; i++) {
    if (ans[i] != '!') continue;
    for (char j = 'a'; j <= 'z'; j++) {
      if (j != s1[i] && j != s2[i]) {
        ans[i] = j;
        break;
      }
    }
  }
  for (int i = 0; i < n; i++) cout << ans[i];
}