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 totally_diff(char x, char y) {
  for (char ch = 'a'; ch <= 'z'; ch++) {
    if (ch != x && ch != y) return ch;
  }
}
int main() {
  int n, t;
  cin >> n >> t;
  string a, b, ans;
  cin >> a >> b;
  ans.resize(n);
  vector<int> same, diff;
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i])
      same.push_back(i);
    else
      diff.push_back(i);
    ans[i] = totally_diff(a[i], b[i]);
  }
  for (int i = 0; i < n - t; i++) {
    if (!same.empty()) {
      ans[same.back()] = a[same.back()];
      same.pop_back();
    } else if (diff.size() >= 2) {
      ans[diff.back()] = a[diff.back()];
      diff.pop_back();
      ans[diff.back()] = b[diff.back()];
      diff.pop_back();
    } else {
      cout << "-1";
      return 0;
    }
  }
  cout << ans;
}