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 n, t;
string str1, str2;
int main() {
  scanf("%d %d", &n, &t);
  cin >> str1 >> str2;
  int l1 = n - t, l2 = n - t;
  string ans;
  ans.resize(n);
  for (int i = 0; i < n; i++) {
    if (l1 == 0 && l2 == 0) break;
    if (str1[i] == str2[i]) {
      ans[i] = str1[i];
      l1--;
      l2--;
    }
  }
  if (l1 == 0 && l2 == 0) {
    for (int i = 0; i < n; i++) {
      if (ans[i] >= 'a' && ans[i] <= 'z') continue;
      for (int j = 'a'; j <= 'z'; j++) {
        if (j != str1[i] && j != str2[i]) {
          ans[i] = j;
          break;
        }
      }
    }
  } else {
    int state = 0;
    for (int i = 0; i < n; i++) {
      if (l1 == 0 && l2 == 0) {
        if (ans[i] >= 'a' && ans[i] <= 'z') continue;
        for (int j = 'a'; j <= 'z'; j++) {
          if (j != str1[i] && j != str2[i]) {
            ans[i] = j;
            break;
          }
        }
      } else {
        if (ans[i] >= 'a' && ans[i] <= 'z') continue;
        if (state == 0) {
          l1--;
          ans[i] = str1[i];
          state = 1;
        } else {
          l2--;
          ans[i] = str2[i];
          state = 0;
        }
      }
    }
  }
  if (l1 > 0 || l2 > 0)
    printf("-1\n");
  else
    cout << ans << endl;
}