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;
const int maxn = 1e5 + 1000;
const int MAX = 2e5 + 10;
using namespace std;
string a, b;
int n, t;
char dif(char x, char y) {
  char i;
  for (i = 'a'; x == i || i == y; i++)
    ;
  return i;
}
int main() {
  cin.tie(0), cin.sync_with_stdio(false);
  cin >> n >> t >> a >> b;
  int d = n;
  string ans;
  vector<int> de;
  for (int i = 0; i < n; i++) {
    ans += dif(a[i], b[i]);
    if (a[i] != b[i]) de.push_back(i);
  }
  for (int i = 0; i < n; i++) {
    if (a[i] == b[i] && d > t) {
      ans[i] = a[i];
      --d;
    }
  }
  bool one = 1;
  for (int i = 1; i < (int)de.size(); i += 2) {
    if (d == t) continue;
    if (one) {
      ans[de[i - 1]] = a[de[i - 1]];
      ans[de[i]] = b[de[i]];
      d--;
    }
  }
  if (d > t) return cout << "-1\n", 0;
  cout << ans << '\n';
  return 0;
}