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;
#pragma GCC optimize("Ofast", "unroll-loops")
const int MAXN = 1e5 * 2 + 42;
char noteq(char c1 = 0, char c2 = 0) {
  for (char i = 'a'; i <= 'z'; i++) {
    if (i != c1 && i != c2) return i;
  }
}
int main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
  int n, t;
  cin >> n >> t;
  string s1, s2, s3;
  cin >> s1 >> s2;
  vector<char> ans(n, '_');
  vector<int> us(n);
  int m = 0;
  for (int i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      m++, us[i] = 1;
    }
  }
  if (m > t * 2) {
    cout << -1;
    return 0;
  }
  int x = 0, y = 0;
  for (int i = 0; i < n; i++) {
    if (!us[i]) continue;
    if (x == y && x + m <= t) {
      ans[i] = noteq(s1[i], s2[i]);
      x++, y++;
    } else {
      if (x == y)
        x++, ans[i] = s2[i];
      else
        y++, ans[i] = s1[i];
    }
    m--;
  }
  for (int i = 0; i < n; i++) {
    if (us[i]) continue;
    if (x == t)
      ans[i] = s1[i];
    else
      x++, y++, ans[i] = noteq(s1[i]);
  }
  for (auto it : ans) cout << it;
}